Replicating lead acid battery with DIY Lithium Pack

themrbruceguy

10 µW
Joined
Jul 21, 2018
Messages
5
Location
Kansas City, MO
Hey there! I am new to posting here, but have been a reader for quite some time (as well as watching way too many YouTube videos on the topic :wink: ) I am very interested in (someday) converting a 60's/70's Honda to electric for commuting to and from work, primarily. But before I tackle that big of a job, I would like to get some practice with something smaller.

I own a 1974 Honda CB350F which is in need of a new battery, and I think this would be a great opportunity to start learning about this hobby. For starters, I know that connecting cells in series increases voltage and that connecting cells in parallel increases amperage. But what I can't seem to understand is how to design the pack in order to match the old battery's cold cranking amps.

Here is the old battery that I am aiming to replicate/replace: https://shoraipower.com/lfx14a5-bs12-backorder--p74

Cold cranking amps (CCA) obviously relates to the pack's ability to discharge current. The reference pack claims 210CCA. Could someone help me understand the math behind building a pack that matches 210CCA?

My best guess is this (correct me if I'm wrong, which is very likely):
  • A configuration of 3s7p gives a 12.6V pack with 21Ah of capacity
  • Each cell (according to the manufacturer's website) is rated at 30A continuous discharge
  • So with 7 cells in parallel, assuming they are all discharging at 30A, yields a total discharge of 210A

Am I thinking about this correctly? Or is that completely wrong?

Thanks! Excited to learn!
~ Jake
 
Have you considered one of these hobby packs.

Are you looking to replace a LFP now or are you bench marking one to build. If the starter spins it up good and easy you can most likely get away with a much smaller LFP pack if replacing a lead brick.
https://hobbyking.com/en_us/zippy-compact-8400mah-4s2p-30c-lifepo4-pack-w-xt90.html?queryID=a9194174078ebb14fbaff5cc810dbad5&objectID=82622&indexName=hbk_live_products_analytics
 
If you need 210 cranking amps, you need to put enough cells in parallel that their total maximum discharge rate is at least 210A. Excess discharge rate capacity doesn't hurt anything, and it might improve starting.

4S lithium iron phosphate cells are a better substitute for lead acid bricks, because their full charging voltages and nominal voltages correspond to each other. If you use most other kinds of lithium based cells in 3S configuration, lead acid chargers and vehicular voltage regulators will send too much voltage. You'll need rigorous battery management to prevent serious problems.

A123 cells have huge discharge capabilities, so those would be a good pick. Headway cells would also work in 4S2P configuration, and they're very easy to assemble into packs.
 
The maximum safe voltage for Li-ion or Lipo is 4.2v/cell. So for 3s, a maximum of 12.6v. A typical lead-acid charging system will go up to 14v, which would be too high for 3s Li-ion. Also, once you put the battery under load, the voltage will drop and the "nominal" voltage during discharge is more like 3.7v/cell, or around 11.1v, which would make for some dim looking headlights.

A better match can be done using LiFePO4 cells in a 4s configuration as Chalo points out, and there are commercial LiFePO4 lead-acid replacements.

LiFePO4 cells can be damaged if allowed to drop below 2v/cell, so some kind of safety circuit or BMS may be needed.
 
speedmd said:
Are you looking to replace a LFP now or are you bench marking one to build. If the starter spins it up good and easy you can most likely get away with a much smaller LFP pack if replacing a lead brick.

I'm replacing a lead acid battery actually; I have never had a non-lead acid battery on my motorcycle. So realistically, all of the batteries I have owned in the past probably put out ~120CCA. But I'm looking at the high-end lithium batteries (like this one -> https://www.revzilla.com/motorcycle/shorai-lithium-battery-lfx14a5-bs12?sku_id=832267) as a reference for my DIY battery pack, which is where the 210CCA target came from.

So you're probably right... Since I'm replacing a lead acid battery with a DIY pack, I really don't need 210CCA and would probably get along just fine with something around 120CCA instead.

speedmd said:
Have you considered one of these hobby packs.
https://hobbyking.com/en_us/zippy-compact-8400mah-4s2p-30c-lifepo4-pack-w-xt90.html?queryID=a9194174078ebb14fbaff5cc810dbad5&objectID=82622&indexName=hbk_live_products_analytics

I have not considered the hobby packs. They look nice, but I am mostly interested in building my own pack for practice and experience rather than cost savings / convenience. Eventually I would like to perform an electric conversion and make my own DIY pack for that, so I thought this little 12V battery pack would be a good starting point to help me understand basic pack construction / safety / etc.

Chalo said:
If you need 210 cranking amps, you need to put enough cells in parallel that their total maximum discharge rate is at least 210A. Excess discharge rate capacity doesn't hurt anything, and it might improve starting.

4S lithium iron phosphate cells are a better substitute for lead acid bricks, because their full charging voltages and nominal voltages correspond to each other. If you use most other kinds of lithium based cells in 3S configuration, lead acid chargers and vehicular voltage regulators will send too much voltage. You'll need rigorous battery management to prevent serious problems.

A123 cells have huge discharge capabilities, so those would be a good pick. Headway cells would also work in 4S2P configuration, and they're very easy to assemble into packs.

Thank you very much for the explanation! That is exactly what I was looking to learn :) And also thank you for the tip on using Lifepo4 cells for this application. Fechter also recommended lifepo4 cells, so I will start looking into those.

Would you mind sharing a link to some A123 cells and also the headway cells that you mentioned? It seems like there are many different geometries / results for A123 and I'm not sure which are preferred or who is a trusted supplier.

These A123 lifepo4 cells with the threaded studs seem very handy (https://www.aliexpress.com/item/100...7.07;-1;10.53@salePrice;USD;search-mainSearch). I'm not skilled at soldering, so something like this is certainly an attractive option. If we assume these cells are what I used for my DIY pack, it looks like a 4s1p configuration would do the trick. 4s will put the nominal voltage at 12.8V. And 1p would allow for 180A maximum continuous discharge (45C max cont. discharge rating x 4Ah capacity... 45 x 4 = 180). Am I interpreting the cell specs correctly? Is it safe to assume that the manufacturer's specs are accurate (the 45C discharge spec in particular)?

Thank you for everyone who has provided input! I am already learning a lot :)

~ Jake
 
themrbruceguy said:
These A123 lifepo4 cells with the threaded studs seem very handy (https://www.aliexpress.com/item/1005003973370056.html). I'm not skilled at soldering, so something like this is certainly an attractive option. If we assume these cells are what I used for my DIY pack, it looks like a 4s1p configuration would do the trick. 4s will put the nominal voltage at 12.8V. And 1p would allow for 180A maximum continuous discharge (45C max cont. discharge rating x 4Ah capacity... 45 x 4 = 180). Am I interpreting the cell specs correctly? Is it safe to assume that the manufacturer's specs are accurate (the 45C discharge spec in particular)?

If those are genuine A123 and not counterfeit, then the specs are probably accurate (but also hard on the cells). It would be pretty amazing to have an effective starting battery that measures 1-1/4 x 5 x 5". But at only 4Ah, you would not have to leave the lights on for long before you squeezed all the juice out. That would make it important to have a BMS to intervene before the cells are overdischarged. For a starting battery, you'd connect the BMS output to the control terminals of a high amp contactor on the + cable. That's more complicated, expensive, and bulky than just a pack of 4 cells.

These cells aren't nearly as small, but they'd be a good cheap way to get your feet wet.
 
Very cool, thank you for the link. I'd like to incorporate a BMS into this pack simply because my future pack (for the electric motorcycle conversion) will surely have one as well. Here is my best guess at a bill of materials for this 12V lifepo4 pack:

What else is there to add to that list? I am also open to recommendations on different sources for those parts listed above. Right now I'm just trying to get an understanding of everything that needs to be ordered.

Thanks!
~ Jake
 
The battery you linked are LiFeP04 pouch cells in a 4s configuration. I had one from that company quite some time ago and it ended up swelling to the point where the case broke (my fault, not the batteries) so I was able to see inside and the guts are nicely built, even if they are expensive and painfully low capacity.

The previous responses have all had good info so I don't need to put in my thoughts as they won't be any different. Just be sure you know exactly how to care for the battery before building and installing it. These don't respond the same way traditional batteries do and you too, like me, could end up with a dead, bulging, or even exploding battery before you know it.
 
I'm replacing a lead acid battery actually; I have never had a non-lead acid battery on my motorcycle.

Lead acid amp hour rating is for full discharge over 20 hours and have a C rate of c/20 while Li cells are rated in straight C rate which is how many times it can be fully discharge in 1 hour. A 30C like the Life one you listed as a bench mark can discharge its full load in 2 minutes without damage. The Cold cranking amps is a bit odd also. Its rated at freezing temp and for only 30 seconds and voltage is allowed to sag significantly during the discharge. If you don't plan on using it in cold temps, it will allow you to use a significantly smaller Li pack. As Chalo mentioned before, you can parallel them as needed to get the current you need. With out measuring current flow to the starter and seeing what it is typically drawing, it is hard to say how small you can go.

No need for BMS if you keep it balanced and all you need to keep it from draining too far is a low voltage cutoff that disconnects it from the ignition. That will keep you from killing it when you leave the key or lights turned on. A typical 4 s balance charger will take care of balance but a active cell balancer would be a good way to maintain it long term if it starts to drift. A BMS is a good exercise to build in but not mandatory and adds a failure point.
 
speedmd said:
No need for BMS if you keep it balanced and all you need to keep it from draining too far is a low voltage cutoff that disconnects it from the ignition. That will keep you from killing it when you leave the key or lights turned on. A typical 4 s balance charger will take care of balance but a active cell balancer would be a good way to maintain it long term if it starts to drift. A BMS is a good exercise to build in but not mandatory and adds a failure point.

Thank you for all of the helpful information! A few questions below if you don't mind:

  • Would you mind helping providing recommended links for the following parts (given my application)?
    • Low voltage cutoff that would work with those 4s Headway lifepo4 cells.
    • 4s balance charger that will charge a 4s lifepo4 pack.
    • Active cell balancer for a 4s lifepo4 pack.
  • In short, what is the difference between a BMS and an active cell balancer? Because I believe the BMS does the same thing that an active cell balancer does but simply has more functions, is that correct? If so, then is the main benefit of using an active cell balancer 1) simplicity, and 2) lower cost?
  • Just to check (if I do end up purchasing a BMS for sake of experience), is the BMS that I linked yesterday (the ebay link) a good option for my hypothetical pack? If it is too overkill / if there are better options, I would really appreciate getting a link for that. I have been learning that the pack doesn't need to be balanced while in discharge, which would save some money on buying a big high-amperage BMS.

Sorry for so many questions! But I am making progress in my knowledge :) Thank you guys a ton so far!
 
Great project. You can make it as complex as you wish to dig into it a bit deeper.

Any of the basic dc hobby chargers should keep it topped up evenly with the minimum amount of trouble. https://hobbyking.com/en_us/imax-b6-dc-charger-5a-50w-copy.html?queryID=6f725e850fdab940d97223718d7706bd&objectID=53604&indexName=hbk_live_products_analytics
https://www.ebay.com/itm/185217050994?hash=item2b1fcbe972:g:vg4AAOSwFmNhu0aR
https://www.ebay.com/itm/144196020889?hash=item2192c0aa99:g:AkoAAOSwD75hPGtC
https://www.ebay.com/itm/283578873786?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

Just add a regulated power supply big enough or a 12v car battery or charger to power it.
You will most likely not need to use it again once installed and trickled with the bikes regulator.
You will want to know how high the bikes voltage goes (reved) so you don't potentially hurt the pack.

Only other things to worry about are cells draining at different rates or a hard short. You can add a low alarm or a cutoff to keep you away from too low a voltage. The alarm will let you ride a bit while the cutoff may kill it depending on how the cb is setup.
https://www.ebay.com/itm/203610602406?hash=item2f68233ba6:g:smYAAOSwJ1FhRpsjqueryID=a75ad0f64ca8046ff9cbc0ead53b62a3&objectID=35827&indexName=hbk_live_products_analytics
https://www.ebay.com/itm/115334913446?epid=2313477426&hash=item1ada7f1da6:g:qNkAAOSwe9ZiUODn

One of these is always a good idea.
https://www.ebay.com/itm/224464607010?hash=item3443221722:g:-RYAAOSwkQVgof5c
https://www.ebay.com/itm/115338585406?hash=item1adab7253e:g:3jIAAOSw0T9iVAwi
https://www.ebay.com/itm/393602515110?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D238115%26meid%3Da89e6d5013504ddf8db5787ec26dc5c6%26pid%3D101195%26rk%3D1%26rkt%3D12%26sd%3D164271864647%26itm%3D393602515110%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv9PairwiseWebMskuAspectsV202110NoVariantSeed&_trksid=p2047675.c101195.m1851&amdata=cksum%3A393602515110a89e6d5013504ddf8db5787ec26dc5c6%7Cenc%3AAQAGAAABIIIp3bTCXFAKw6S9crk9CpCrrtOgadqFiC10NVPbN0936eprYVB4SXfDa52e1mbPLBSV6nTTucuMgv3YRmt7KRdKPaOFUO0P0dVueDJHBQOMiOs4GVAY5s96CtLzgarft3UKbcDvY22xETap1Ox6bsx%252FrbRq%252FpYENs9jCe1csQEpg9IF3ggQIz60bvl6Eg4QZbRUPrjy6XrzoAWoE9za3KrBstYrCZRTY%252BSuf0VCXfUB%252Fwnd5NZwdwMSKMcRtDsLY32vLVFV9n272ox786BQvRKfn0kduvrhmh6Mv86ONOhItCnQ2G6qMFmBNMotCrjo0aW2LjhbXfXa0OpH46%252BgGMpigpW4iITphLD%252Fw2cNpgPUk8t4CRa5XoxVsLg5UKnugA%253D%253D%7Campid%3APL_CLK%7Cclp%3A2047675&epid=5039133645

https://www.ebay.com/itm/164271864647?hash=item263f5dbb47:g:a7MAAOSwRNle~U-8
:)
 
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