[PSUBS-MAILIST] Lithium Batteries

Fri Apr 10 06:38:44 EDT 2020

Thanks Antoine,
yes some problems there. Agreed there would be a ridiculous number of balance wires 
coming out of the battery pod on a large pack unless you had the bms inside the pod.
I can remember an inexpensive PCB they were using on each battery in a DIY
electric car conversion. Will have to do a bit of Googling on that one.
I split a 10s 36V pack in to two 5s lots for charging so I could use my inexpensive
charger. So maybe there will be a solution along those lines.
Alan

> On 10/04/2020, at 8:59 PM, Antoine Delafargue via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> also, 
> another cost issue with lithium ion batteries (incl LiFePO4) for our applications is the Battery Management System (bms). 
> You cannot just add batteries in series and parallel like with lead acid, because you need to control the voltage of each cell very accurately to avoid over charging or one faulty cell debalancing/shorting the others, and to avoid too brutal cells charging after prolonged rest, all of which could lead to fires. The bms is also needed to manage cell balancing at the end of the charges to bring up cells lagging in voltage up to full charge whilst preventing the faster charging cells to exceed safe maximum voltage. This requires extra cabling to purposely bleed off the faster charging cells. 
> 
> So you have several options:
> -go with a fat battery with a built in bms. but that limits the amount of kWh stored (in which case you may be better off with lead acid as weight is not an issue in our subs), and constrains flexibility on dimensions. 
> -use certain models which have built in bms allowing for a certain pairing in series/parallel. but those may be quite expensive as these are fancy features and they duplicate electronics in each battery.
> -make a dedicated pack and bms. Having a professional do a tailored bms can double the cost of your battery. There are probably open source bms architectures available out there (from the solar home systems, survivalist, mountain biking etc. communities). 
> 
> regards,
> Antoine
>  
> 
> 
> 	Garanti sans virus. www.avg.com
> 
>> On Fri, Apr 10, 2020 at 3:10 AM River Dolfi via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>> Alan,
>> I've seen the price comparisons that you're referencing. They're based on total cost of the lithium bank distributed over a "lifetime" of 3000 full discharges or so. After 3000 cycles, the batteries are at 80% of the original capacity. That is what they constitute as "failure". 
>> The problem is, most psubs, and most professional subs for that matter don't come close to 3000 total battery cycles. That makes it hard for most people to justify the initial purchase price.
>> 
>> -River J. Dolfi
>> 
>> 412-997-2526
>> rdolfi7 at gmail.com
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