Lightweight battery

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robin
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Re: Lightweight battery

Post by robin » Mon Sep 24, 2018 8:13 am

The problem with Lithium Ion type batteries (this includes Li Ion, Li Polymer, LiFePO4, etc.) Is that cells are destroyed if allowed to discharge below a certain point and they also have a narrow operating temperature range compared to lead acid. It's usual to include a control circuit to cut off the battery when it approaches deep discharge, but as these involve putting transistors (big FETs in this case) in the path of the current, that presents a challenge for handling the starting currents - your isolation FETs have to be able to conduct 200A+ in normal circumstances. Assuming your battery is made up of 4x3.2 cells in series (x some number in parallel) then the cut off is at at 8.0v - a voltage that can easily be hit during cranking if the cells aren't very well charged. At 8.0v at least one cell is <2.0v which is the death point for a LiFePO4.

Now what happens is that they don't all fire in parallel - some will cut off a bit sooner than others - and so as each parallel bank drops out the load is presented to the remaining ones. When you get down to the last one, it's seeing 200A (much more than it can tolerate), for sure it will dip below the cut off voltage and then its FET will try to go open circuit to break the current path - unfortunately the chances of what is probably a 20A FET shutting off a 200A current without melting on the way is very very low. Now there are engineering solutions to these problems, for example:

* Connecting all the raw cells to a single isolation circuit - but this has other problems - the parallel tiers are now not isolated from one another and this causes problems during charging (unbalanced charge) which in turn creates a high self discharge current the rest of the time. It's all too easy to end up with a battery that has a very low effective capacity, much lower than the sum of its cells.

* Allowing each cell to be its own battery and connect them all in parallel, then use electronics to convert the voltages to/from the nominal vehicle 12V; but this is very inefficient and you need a 3.7v -> 12V boost circuit that can deliver 200A - this will be BIG.

* Just don't have any cut off at all and include a statement like this in your user guide:
Make sure the battery does not get deeply discharged. If the battery has no charge remaining when you check it, consider it to be damaged. Do not attempt to recharge or use it.


A good manufacturer will carefully match cells for capacity and internal resistance so as to minimise the challenges of building a large batteries out of many cells. A bad manufacturer will just chuck any old cells together - it'll work day 1 but might not survive its first deep discharge event.

Bobby, the battery you selected is LiFePO4, which is the best of a bad bunch - it's a chemistry that is suited for EV applications due to having high current handling capacity (you can often charge/discharge them briefly at 10x their nominal current rating, which is good for cranking). Whether or not there are any electronics in the battery is hard to tell from the outside. I am surprised you need a charger - I would have thought the vehicle alternator should easily charge the battery; but maybe the charger is really a device to prevent discharge when the vehicle isn't being used?

Anyway, will be interested to see how you get on ... if the thing ends up dead, I would like to take a look inside it if you are out of warranty - I have a passing interest in batteries for EVs.

Cheers,
Robin
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H8OAG
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Re: Lightweight battery

Post by H8OAG » Mon Sep 24, 2018 10:33 am

Amazing Robin........a good read

:P

A lot of the (MG)race boys are using these batteries for weight saving, but their unreliability is such that they are treated as throwaway items when they go faulty.

I shall try and procure you one for your research Robin

:cheers
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C7Steve
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Re: Lightweight battery

Post by C7Steve » Mon Sep 24, 2018 12:13 pm

Welcome ‘back’ Robin. It’s great to have your technical knowledge again. :thumbsup


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Sanjøy
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Re: Lightweight battery

Post by Sanjøy » Mon Sep 24, 2018 10:15 pm

Damn it Robin I was going to post that.
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rawsco
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Re: Lightweight battery

Post by rawsco » Tue Sep 25, 2018 7:29 am

Good to see you back Robin and a very interesting read
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campbell
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Re: Lightweight battery

Post by campbell » Tue Sep 25, 2018 12:02 pm

Think I'll stick with lead acid

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robin
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Re: Lightweight battery

Post by robin » Wed Sep 26, 2018 7:08 am

Actually, if you want a long life battery for your elise then marine cells are ideal - assuming they can be made to fit - they are typically gel filled rather than liquid electrolyte and they survive deep discharge, cold weather, etc.
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Bobby
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Re: Lightweight battery

Post by Bobby » Wed Sep 26, 2018 6:16 pm

Very in-depth Robin! I was somewhat reticent on Lithium batteries, as I'd heard a lot of concerns. I did my homework and in the end, I thought many of them could be put down to lack of care and mistakenly treating them the same as a traditional battery, so decided the only way to find out for sure was to try it and see!

Deep discharge is clearly an issue. If you leave the car for a long period with battery connected, alarm etc. on and draining, it's going to be a problem. That was the rationale for the charger, so I don't have to worry or faff about with disconnecting. One thing I notice on the charger it that it has a optimisation feature, that seems to address some of the cell balancing issue you mention. It also has some rescue programmes.

So far I've no issues. Survived a round of SLS, a trackday and some general outings, all with a lot of stop-start. However, I've not done any long trips where it would be off the charger for a prolonged period. I've no idea how long normal drains will take before they become an issue. Makes me reticent to set the alarm, for sure!

I got the same impression you noted that there seem to be a lot of people out there selling these, but quality may vary; some are clearly small operators bolting together components with questionable expertise. I'd certainly advise going for well-established suppliers.

The Varley one clearly has a bunch of electronics in it from the doc's, but unclear what exactly. Wherever they employ, they quote a 5 sec sustained current of 670A, peak 1470A (although if that is hit, you've probably got other problems!)

I hoping it doesn't turn out to be a throwaway item, as they're not cheap. However, if you are looking for weight saving, I think they are one of the better options in terms of £ per Kg!
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