Beyond Unboxing: How to Take Apart a Ford Fusion Hybrid Battery While Minimizing Death for You and Onlookers

Wednesday, November 13th, 2013 @ 17:40 | Beyond Unboxing, Stuff

Alright everyone, it’s time to get serious! This is the first of most likely several engineering posts directly relevant to Chibi-Mikuvan (see its introductory post here), in which I’ll delve into the parts that I’m intending on using in the build.

No, I was not kidding one bit.

・ω ・

It’s the hallmark of this whole thing.

Anyways, recall that most of my recent EV-related builds are in the interest of pathfinding and trailblazing. Taking a stab in the dark at a potential useful part for makers and builders and seeing how it works out, some times involving a little bit of science, and possibly incorporating it into my personal go-kart kindergarten. That, and ultimately looking silly. With this build nominally trying to fit under (or at the least, not outrageously around) the PPPRS $500 parts and materials budget, I’m intending to try several new tricks at once, and all of them will be exhibited here.

This post in particular will focus on the one pivotal strategy that I explained in the post-Maker Faire post – cracking apart a second generation hybrid car traction battery and using the internal cell modules. These modules are typically NiMh chemistry; this was before everyone switched to lithium ion batteries this and last year. NiMh lends itself to highly favorable energy density over lead-acid batteries. Being several years and tens of thousands of miles out, you can find these batteries sold by auto recyclers for between $300 to $500. For people looking for huge capacity in less weight, I think the cost per watt-hour of energy of a used hybrid car battery is the best you can get.

In fact, my final cost in this investigation was $300 per 2.15kWh, or $0.139 per Wh. This beats out even Hobbyking lithium polymer batteries at 33-50 cents per Wh (which aren’t allowed in the first place in the series!) and most lead-acid battery options short of ‘found objects’ or purchasing at heavy discount. This is now on record as being possible, which, really, satisfies me even if I don’t end up build my own entry with it.

Someone else can totally take the torch from me right now! I’m certain somebody’s already dropped a hybrid battery into a custom project, but this is where I declare pics or it didn’t happen. Otherwise, feel free to scoop me on actually using these, because I’m sure I’ll never get around to it at this rate.

So here’s how it went down:

Initial reseach

The first step was hopping around on Wikipedia, hybrid owner forums, and Google Images compiling a list of battery candidates. I had a few in mind after a day’s worth of looking around:

  1. Toyota Prius, 2003-2009, or the “Gen 2″. This was the duh option – millions of these are on the road, so their parts are bound to be the most plentiful. The prius battery is a block of 28 “blades” composed of 7.2v modules, each 6.5Ah. The total battery energy of the car is  302V and 1300Wh (total, not useful – there’s a distinction). I was particularly interested in the Prius at first because the 28 blades could be arranged into a 28.8v, 45.5Ah battery by putting four 7.2v modules in series, and seven of those in parallel. 28.8v happens to be near the maximum voltage rating of many inexpensive R/C power system options.Along with the Prius, other same-gen hybrids like the Camry Hybrid and Toyota Highlander hybrid share the power system.
  2. Ford Fusion Hybrid, 2009-2012. This option was familiar to me because the MIT EVT chopped the powertrain out of a 2009 Mercury Milan Hybrid and converted it to full battery electric. The car’s advertised battery is 275V, 2.1kWh. We took apart the large hybrid module about 2 years ago, long before I had any delusions of using it on a small go-kart (since we were, and still are, up to our nostrils in donated A123 cells). I knew already that this battery was composed of 28 9.6v, 8Ah NiMh D cell modules, which can be arranged into a 28.8v  72Ah battery. Similar year Mercury Milans and Lincoln MKZ share the same powertrain.I also briefly looked at the older Ford Escape hybrids (2004 to 2008 years) which used a higher voltage, lower capacity (330v 5.5Ah) 1.8kWh battery, but I did not find many for sale, and the familiarity with the Fusion pack meant I focused on it harder.
  3. Honda Insight and Civic Hybrid, first generation, 2000-2006. I knew little about these except that they had 7.2v, 6.5Ah modules arranged in a 144V battery pack, which means they came in at a clean 1kWh. These are getting rare, however, and I did not find many for sale that weren’t very expensive.

I found an interesting price curve when studying eBay’s listings. Well known hybrids like the Prius commanded high prices because you know people are going to need parts, and will pay for them. Rare hybrids like the Insight command high prices because of the few people who are still keeping them on the road. It was the hybrids that nobody really cared about, like the Civic Hybrid and Fusion Hybrid (and relatives) which seemed even remotely plausible. Since I knew what was coming in the Ford Fusion battery, I decided to focus harder on it, and still keep options open by inquiring about the Prius.

Purchasing

I was aiming to get $300 or less for one of these units. This was the number that I decided would make sense for a PPPRS entry – you take the battery, split it in two to obtain 28.8v and 32Ah so you can have two of them to swap out between, and each battery half will only count for $75 on the budget under their accounting rules: $300 purchase price, $150 budgetary price, and half of that running at any time.

I began by scoping out these batteries on auto yard listing websites such as car-part.com, to identify potential candidates. The plan was to get a handful of leads, then call the yards themselves pretending to be an instructor for a MIT electric vehicle design team whose students are carrying out a project where they want to utilize a hybrid car battery, and would you happen to have any as-is, repairable, or core return batteries we can use?

Only 10% of that sentence was bald-faced lying through my teeth, so I felt clean!  The hope is that someone would drop their price for such a noble cause. You might call this unethical, but it was also a good probe of how open these yard operators were to the idea of makers and hackers (mis)using their parts, especially young ones. If you weren’t actually going to put it on a car, the yard doesn’t have to worry about a warranty or whether or not the part is damaged or not functional to OEM spec. Even a worn out battery from an automotive perspective is most likely still useful for small vehicles. Hence my emphasis when I called on these packs being rejects in some way.

This was an adventurous roughly 2 weeks. I would call to find they have no stock, but be told to “call back next week in case” – so I did. So many of these were “Call this number and ask for Billy” type situations, which I frankly love. Here is a sample of the text file that I was compiling:

Chuckran’s 508 697 6319  x

Robertson’s (800) 551-7000 x

Linder’s 800 521 8000 ($900 07 Prius ) ($700 09 Fusion) Jesse sent you

D Richards Jack 800 776 0459 $300 ! 10 Fusion

Tom’s 800 255 6656 (left my info)

A-rite Auto 800 874 7116 $700

LKQ 800 500 8733 Candia NH call back next week JC

Bishop’s 860 346 2336 x check later

Goyettes 800 640 7548 fax 508 207 1546 x

Self-Serve Used Auto Parts 1-508-763-4442 Check in a week

Route 128 Used Auto Parts 780-980-0025 (calling back soon?)

I ended up calling 18 yards in increasing distances from Boston, gathering prices and references (When a yard didn’t have stock, I asked for a recommendation on other businesses to call). Several of them led me to each other by accident. The price was slowly falling the further I got from the metro area – this made sense, after all, since I’m sure not only the cost of doing business is higher closer to an urban area, but more cars means more wrecks and more throughout and more people needing their car back on the road; all leading to higher prices. I started breaking into Connecticut and New Hampshire.

Ultimately, my lead came from Burlington, Vermont. So one Friday morning a few weeks ago at 5:30AM sharp, I blasted out of Boston and ended up 45 miles from Canada about 4 hours later, collecting no maple syrup or moose in the process.

Or bears. As I wandered further up I-89, the “_____ Crossing” signs got more and more hardcore. Bears? What’s next, T. Rex?

I must say, if I spent my life here, I’d be a nature hippie too. Burlington is pretty damn beautiful, especially down by Lake Champlain.  I wanted to pull down by the waterfront to get a Vans Next to _____ picture (the VT-NY ferry was a good choice), but the area was blocked off and full of tour buses. I decided against pushing my luck.

The yard itself was about 15 minutes north, in a small town called Colchester:

D. Richard Automotive was the name of the place, and I didn’t exactly look out of placed parked amongst their selection of used cars. A few what the hell is that questions were received from some of the customers and staff, to my amusement. Apparently Mitsubishi vans just never quite diffused evenly throughout the country.

The owner, possibly Mr. D. Richard himself, was very receptive to the idea of student-built EVs using these packs, and I made some idle chit-chat with him about 2.00gokart and the Makersphere while his… Ninjas? Cronies? were retrieving the part from stock. It’s a little far to drive every time something good comes up, but more links in the network, the better!

This is the thing. If they made Hybrid Mikuvans back in the day, I’m guessing the battery will go in a not dissimilar position in the rear. It weighs about 150 pounds (judging by my non-ability to lift it). Check out those bullet connectors.

A four hour drive back home and some Bro Assist got this patient on the operating table. The T-square is to illustrate how big this is – about 20″ wide by nearly 4 feet long. By this time, I was pretty much worn out from driving totally randomly into 3 different states to get a part for a silly go-kart, so I passed out. The following pictures were taken some time after the fact.

That thing above the battery is a cooling fan that pulls air through the cells and exhausts it to the side.

These are the side vents in the case, through which you can see the cell modules.

The fan shroud is held on by a handful of small hex head bolts. Use a 8mm socket (5/16″ works too) to remove them.

Before I totally took the fan shroud off, I decided to have a look at the fan itself, since it’s a brushless fan for long life and greater efficiency. What was it – a really hardcore Hobbyking controller driving it? Could I turn it into a hub motor?!

The fan pops out with the same little bolts, three of them. Under the big white sticker are 4 little Phillips head screws, and after you remove that:

Holy hell, it is a Hobbyking controller! A really, really hardcore one…

This thing appears to have its own DC-DC converter in line with the power input. Judging by the 35V output capacitor, it probably modulates the speed of the fan only through this DC voltage, either raising it past 12V or lowering it, commutating the phases fully on or fully off at all times. This architecture is called a current-fed inverter, and it saves you from having to PWM all the 3 phase bridge FETs, cutting some losses and complexity.  I would have literally lobbed a HK controller in this, startup beep and all. I guess this is why I am not working for a car company.

(If I had it my way, everything would have a V8 made of Melons in it.)

Once you pop the retaining ring on the back side off, the rotor dismounts:

It’s a copier motor.

I literally burst out laughing uncontrollably when I found it. It is the exact same dimensions as a typical 68mm copier motor I’d harvest from some poor Xerox to use in a scooter hub motor. Hey, this winding might even be fine as-is for a hub motor, since it’s a fairly high voltage motor!

Some larger Torx screws also hold the lower fan shroud on. A T25 Torx bit is the proper way to attack these. With the fan shroud tossed aside, it was time to start cracking the battery itself open.

Here’s where I pause for a second to say:

The following images depict opening up and messing with a 275 volt DC power system. It will be exposed, be unfused, and be extremely dangerous. If you touch the wrong spots, you will commit suicide instantly. If your metal tool touches the wrong spots, it will explode in a plasma cloud and deposit itself on your face. It is your responsibility to educate yourself on proper handling of, and safety-oriented behavior around, high voltage systems.

tl;dr be safe around batteries, kids. These pictures are for your reference and educational value only – I am not here to train HV electricians. As you’ll see, the battery pack is extremely well designed and anticipates service and disassembly, but it makes best practices no less necessary.

Now, onto removing the battery’s shell.

First, I had to remove the various other warts and boxes on the bottom of the battery. These include, from the left to the right, the main contactor set and output connectors, the orange  service disconnect, the battery charge monitor controller, and the battery computer which interfaces with the rest of the car.

These are all held on with nuts on studs, and I used a 12mm deep socket to reach them.

I decided to ruin the resale value of these OEM battery parts by prying them open and looking inside. (Seriously, I could have made $150 on each of these battery controller parts!)

This is the “Battery computer”, as it seems to be called . I’m assuming this takes battery voltage info from the other box (with the HV interfacing hardware) and pipes it to the car’s ECU. I’m not focusing on reverse engineering this stuff, so it’s here for visual stimulation purposes.

 The other box contains the high voltage interfacing hardware – check out that isolation gap! My guess is this has an array of multiplexers and ADCs to sample the battery voltages at the module level. That or a huge string of op-amp subtractors, Hobbyking style, with 400 volt opamps?

Some more prying and biting later:

Looks like the two theories might be combined. On the left board (the first module) are what appear to be several 14-pin, possible op-amp or comparator ICs. The part numbers are all proprietary – even though they feature logos like NXP and Texas Instruments, the etched numbers all seem to be serial numbers of some sort. The myriad passives on top and on bottom suggest to me a possible series of op-amp circuits.  The right board has a shoreline made of optocoupler solid state relay devices, KAQW216 (the chips are marked W216HC5). Most of them seem to eventually pour into (or out of?)  a large Analog Devices chip, an ADUM1401 4 channel digital isolator, surrounded by some quad op amps (AD8554).

Again, I’m not out to decompose the battery management system, since I won’t be using it and it’s surely very proprietary, so this is as far as I’ll go here – component staring and hypothesizing. My fan theory is that the 4 channel isolator turns on the solid state relays in a multiplexer fashion to select groups of cell modules, the top end and low end voltage of which flow into the op amps to be subtracted to get a voltage number; combinations of groups of modules being read will let you deduce the voltage of each individual module if you keep track of what is being read at any one time.

Moving on:

This is the contactor bank of the battery pack. There’s three separate contactors: one for positive, one for precharge (connected to positive), and one for negative. In vehicles, traction batteries are generally fully isolated when the vehicle is at rest. It’s not frame grounded like you would do for a 12V auxiliary system. It’s great to see all the systems my students typically build into a small or medium size vehicle (like the precharge circuitry) because I tell them to, being used on an industrial scale because you neeeed it in order for the system not to grenade instantly.

The set of two contactors at the top left control positive – one of them fills the HV rail through the precharge resistor (the big sand block resistor next to them). The single contactor at the bottom is the negative. The only thing that looks a little cheesy is the great big negative bus bar that spans the entire length of the thing.

Finally, the black hole that the positive busbar disappears into is a Hall Effect current sensor which, again, seems to be a proprietary part. Maybe I will take it apart in more detail and try to extract the functionality later.

Update:  The current sensor is a LEM DHAB-S/34 type, which outputs an analog voltage proportional to current. It has dual sensitivities – one pin for 40mV/A, the other for 10mV/A, so you can monitor low currents and high currents alike while minimizing quantization error and sensor accuracy error.

I decided to remove the back cover first to check if anything else needs to be removed. More 12mm nuts and T25 headed screws hold this on.

The yolk of this egg is beginning to show. Twenty-eight D cell modules, bound together by (non electrical) aluminum bars and plates.

I came around to the front again to disengage the service disconnect. Without this connector in, there is no voltage present on the contactor pack. I had to thread it through its panel hole to release the cells eventually, however. This also meant disconnecting the large ring terminals that fasten the battery to the contactor pack first – they’re at the very top, isolated by a plastic wall to prevent “mishaps”. A 14mm nut holds the terminals together, so I used a deep 14mm socket to extract them.

YEP, IT’S A BATTERY ALRIGHT

266 volts from non-isolated negative (before the contactor) to the service disconnect.

After I removed the contactor pack and service disconnect, I needed to remove the handles to gain access to the side plates. These handles are held on by some 12mm hex headed bolts that also retain the upper two sets of structural aluminum bars.

The bottom set of bars have giant Torx screws holding them in – T45 to be precise. I only had this size in a ratchet drive.

And both the side plates pop off…

There it is – the delicious Roe of Fusion at the center. But we’re not done yet – I still need to take apart the pack module by module.

The battery modules are retained by these non-electrical aluminum bars (I keep wanting to call them busbars, dammit!). So really, I could have just removed both top and bottom cover (and contactor pack), then only one side plate removal was needed. I could slide the whole thing around at this point.

Little temp probes that contact the cells at certain distributed locations to get a pack temperature reading.

Some intern had a lot of fun designing this engraving, I bet. This bar is totally something you’d hand off to an intern to do. It’s a straight piece of metal with holes in it! There is no way it’s fuckup-able!

Here’s how the pack comes apart. The moment you take the end module off, the whole pack is split into two, so you can’t even drop something across the primary positive and negative. Furthermore, you can only remove one module at a time: the busbar retainers interlock and are retained by two adjacent modules, so you must necessarily disconnect a whole module and remove it before moving onto the next set.

This construction is quite ingenious and must have taken weeks if not months of engineering committee meetings to come up with. I and everyone present at the first autopsy of this battery have said that it’s best example of design-for-assembly and design-for-service that we’ve seen.

I would never, ever have the patience to design such a system.

The back side of the battery is one solid strip of bus bar endcaps, removed one at a time at your leisure. The module endcaps are M5 threaded, and all the heads are T25. Someone really, really loves T25.

The full disassembly would look like a messier version of this – a small mountain of cell modules being slid out one at a time from the main pack.

The busbars live in snap-in mounts and are 1mm thick copper. On the “front side” of the battery, they simply are friction fit in place – they slide right out.

On the other side, they are retained by little snap fits. Unsnap them to take them out if you want to reuse them.

THIS IS THE BUSINESS END OF THE PACK. To avoid having to deal with this at 275 volts, take apart the pack starting from the right side of the contactor pack, if you’re looking straight at it. That’s the way I have it done here. If you start from the left side nearest to the contactor pack, you get to disassemble 275 volts first. Have fun.

If even one module is removed from the right side, this connection is isolated.

The sticks are made of 4.8 volt strings of D cells, with (I think) a nominal capacity of 8Ah. As you charge and discharge them faster, the useful energy will decrease – I put these on an aggressive 1 hour charge, so they won’t read 8Ah. One that I left overnight on a slow near-trickle ended up at 8250mAh.

Here they are inside. The cells are held off from their cases by rubber insulators; this is so air can flow through the case and around the cells, allowing them to cool or heat using the fan.

I may delve into the contactor pack in a future post. I think it’s quite useful stock and as-is, so I did not take it apart. After all, it’s just contactors with wiring already built in, and a precharge to boot – it doesn’t care if it’s being run at 275 or 27.5V – the coils are all 12 volts.

The contactors are nice – way better than the cheesy relays the students rig up remote-power circuits with in my class (but then again, that’s all they need). They’re Matsushita (Panasonic) type AEVS contactors designed specifically for EVs. It’s hydrogen filled!… wait, isn’t that bad?! Hindenburg Relays!

I’m sure if you had to buy these they’re a hundred dollars or more alone, but you get three of them with purchase of a cheesy old hybrid pack. Limited time offer.

Since I did not need this pack immediately, I decided to not take all the modules apart. Instead, I put the end plates back on so the whole thing is rigid, and it currently lives on a handtruck in the shop. I left the rightmost ‘crossing busbar’ out so there’s nothing 275 volts about it in public.

I made an almost-accurate model of the Fusion Sticks and have uploaded it on my reference page as a Parasolid (x_t) file, which should import into most CAD programs. The dimensions are accurate to 0.1mm for the first inch of length on both sides. There’s a lot of little randomness that sticks up in the middle, including the aluminum structrual bar channels, that I didn’t model since I was planning on mounting them on their ends only in my application. You could, if you’re crafty, just saw the bars in half and arrange the stock copper pieces to your liking on the cells, then redrill the end holes and use the stock endplate and handles.

Each module weighs 3.66 pounds, so this full pack of 28 is 100 pounds. And 2.1kWh! Beat that in lead.

So there we have it – another episode of Charles Takes Something Apart for His Own Amusement at Great Personal Expense Beyond Unboxing! I would actually love to do a series of this just on hybrid car packs, but it might have to wait until I find another one such as a Prius module or Leaf lithium ion module for super cheap (or someone comes and drops one on my desk). Mass produced commercial things always have the potential to be low cost and highly functional, and I think the gradual increase in market share of hybrid and electric cars is a boon for the hobbyist or home builder.

 

Recently

  • Chibi-Mikuvan & The New York Maker Faire 2014 Recap
  • Dragon Con 2014: The Wrapup, or, Operation: I FEEL GASSY, plus Panel Resources
  • The Dragon Con 2014 All-Robots Update
  • Silly Go-Kart Design: The 2014 Summer Season
  • Chibi-Mikuvan: Detroit Recap and The Conclusion
  • Chibi-Mikuvan: The Penultimate Update!
  • Chibi-Mikuvan: Getting “Tired” of All This; Making the HCL Actually Function; Plus Other Work
  • Chibi-Mikuvan: Experimenting with the Trackstar 200A and the Hysterical Current Limiter
  • Beyond Unboxing: The Great Cambridge Chainsaw Massacre; Ryobi RY40511 Cordless Chainsaw
  • Chibi-Mikuvan The Extended Universe Edition: Water Cooling Loop and Digital Dash
  •  

    9 Responses to “Beyond Unboxing: How to Take Apart a Ford Fusion Hybrid Battery While Minimizing Death for You and Onlookers”

    1. Max Says:

      This thing scares the #@%^Y@! out of me, which is probably a healthy reaction although a hardly useful one – I’m not intending to ever mess with one of these. That’s not saying much though, ordinary 12V car batteries also scare the #@%^Y@! out of me: it’s something about my aversion to the whole process of high-current induced deposition of plasma-phase metallic particles on skin surfaces. I do definitely enjoy seeing them being “unboxed” (beyond) though, so thanks for that… :)

    2. Dane Says:

      I really like the integrated cooling design of the module, and the whole ‘design for assembly’, the modules appear to just lock in, there’s no way to install things backwards. Have you run across a Datasheet for the cells?

      -Dane

    3. Marek Says:

      I love the teardown! Thank you for your work and sharing. I just have a question – this pack has 2.1 kWh of energy, so it is equivalent to about ~ 5 x 12 V, 40 Ah lead-acid car batteries. Why not to use those? I am thinking about a conversion of a small car into electric, but it seems that there is no new way to put significantly more energy in. I guess I could put 2 – 3 batteries like yours in the trunk / engine bay, which beats lead acid, but other than that it is not that better. Thanks again!

    4. the chuxxor Says:

      Energy density. This whole endeavor is to see if these batteries are a good alternative to having to haul a pile of lead around in a smaller vehicle.

      For instance, the same 5 lead batteries you mention would weigh around twice as much. Car batteries are also not capable of repeated deep-cycle discharging.

    5. Dro Says:

      The Chuxxor: Can you clarify the following:

      Here’s how the pack comes apart. The moment you take the end module off, the whole pack is split into two, so you can’t even drop something across the primary positive and negative. Furthermore, you can only remove one module at a time: the busbar retainers interlock and are retained by two adjacent modules, so you must necessarily disconnect a whole module and remove it before moving onto the next set.

      Is the voltage on those terminals just the voltage of one module, or is it half of the 266V? Did you have to use an insulated screwdriver or something? I am guessing that either the potential across the exposed screws are just the voltage of a single module or are decreasing as you take each module out?

      It looks like these batteries are fairly benign once you get them all taken apart, and can be used to build up a more tame (less voltage) version battery for use in something, removing a large degree of looming shock and/or death potential.

      Excellent post, thank you for going through all the work to document and share the information. Greatly appreciated.

    6. Kj6epl Says:

      Max: While your concern is valid, I think maybe you’re a bit too worried. High current is not as dangerous as you’re making it out to be. If you were to drop a screwdriver across a car battery, you’ll get some impressive sparks, but it will NOT instantly vaporize. Rather it will start glowing after a few seconds and melt; that it all. If you touch the contacts on a 12V Pb battery, likely you won’t even feel a shock. I know ‘cuz I’ve done it; not even the slightest tingle. Per a video from PhotonicInduction on YouTube, you have to be up around 40-50V before you’ll even be able to feel the slightest shock (unless your hands are wet), and up to 100V before the pain gets intolerable. Go check out that guy’s channel if you want to see what really happens in all of the “worst case” electrical scenarios.

      Anyway, I found it interesting that they are using D cells in these packs, and that after 3 years it still held its full rated charge or very close to it. I wonder how a lithium pack (from a Volt or Tesla) would have fared over the same length of time?

    7. the chuxxor Says:

      Hey Dro. When you open the shell, there is only 275V at the very left end by the main discharge leads. On the other side, the module at the very end (which you should remove first) only has its 9.6 volts across it.

      As soon as you remove that module, then the pack is split in half, and with each additional module removed, you take off 4.8v from each ‘side. The only way to cause damage would be to somehow bridge the entire length of a module – almost impossible to do given the shielded nature of the interconnects.

    8. Radu Says:

      Watch out on the quick-charging of NiMH batteries in parallel. Each parallel string can reach peak voltage at different times and confuse the charger’s DeltaV detection. Worse yet, the string that reaches end-of-charge first will dip in voltage first, sending more current to the already-charged string. Charging at super-slow rates like C/20 sucks but it works. One thought is putting a temp sensor on a cell of each parallel string and using that to trigger cutoff (no joke, some of the old RC airplane quick-chargers used temperature spike as the cutoff method)

    9. Bootstrap Says:

      Rad writeup, thanks. To add a bit of info to the SLA vs. NiMh discussion, SLA ratings are typically at 1/20th or 1/10th C, the capacity is much lower if a higher current is drawn e.g. a 38AHr SLA at 0.1C only provides 22AHr at 1C. Also, the final voltage in the above test is so low it would permanently damage the battery. Rule of thumb for SLA capacity can be to take 50% as the usable capacity for long life, and that’s for 0.1C. NiMh ratings however are often at higher discharge rates e.g. 0.2 or 1C, and the final voltage is not so low it would take life off the cell.