Z$M boot battery conversion

S

smedleyrup

New member
Hi
Has anyone ever modified the large space taking boot battery for two smaller units in series mounted in the corners - perhaps lithium ion ones?
I want to travel in the car, don`t want a boot rack and this seems a possible solution?
Regards,
Rupert
 
Lithium Ion batteries aren't charged the same way as lead acid so I think you may fall at the first hurdle unfortunately.
 
ph001 said:
Lithium Ion batteries aren't charged the same way as lead acid so I think you may fall at the first hurdle unfortunately.
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Works fine in mine.
smedleyrup said:
Hi
Has anyone ever modified the large space taking boot battery for two smaller units in series mounted in the corners - perhaps lithium ion ones?
I want to travel in the car, don`t want a boot rack and this seems a possible solution?
Regards,
Rupert
Click to expand...
Yes, you can mount the battery on the bulkhead and buy a couple of pieces of trim from the 3.0si coupe which has the battery under the floor to make it look ok. The problem you will have is that a lithium battery slim enough to do this will need charging quite a bit if you leave the car locked with the alarm on, probably fine for a night or so though I've never tested it with my car. Maybe having a couple in series might help you with this as you suggest.
 
Lithium ion cells are charged at 4.2V / cell (so 12.6V as you will be using a 3 cell pack). Some of the special high capacity packs can go up to 4.3v (12.9v) but the alternator puts out approx 13.8v to 14.3v so even at idle it will be charging the Li-ion at 4.6v per cell.

Li-ion absolutely cannot accept overcharge, so I can only assume that the commercially available car ones have some kind of active regulation inside???

Finally, storing Li-ion at full charge dramatically shortens their life (in stark contrast to lead acid). Ideally they need to be stored at half charge - not easy in a car!
 
That's all a bit inaccurate for most of these lead acid car battery replacements. They generally use LiFePO4 technology which have a nominal 3.2V output so you're looking at 4 cells for 12.8V. As long as the charge doesn't exceed 3.6V they're supposed to be fine from what I understand. It's also quite easy to limit max charge to 85% say, which is what they do in most industrial applications and I presume for the automotive sector. I've had no issues in the last 9 months or so and I know many who have been using them a lot longer than I in similar applications.
eta: to the op, I guess you meant hooking 2 batteries in parallel rather than series, regardless it's apparently really not a good idea with li-ion batteries, nice way to cause a fire it seems.
My battery is only 20ah vs say a normal lead acid of perhaps 80 (?)ah, but it's tiny. You could use a bigger version but you'd then be into modifying the interior pieces really to get it to look half decent I reckon?
 
Ok, if its LiFePO4 he’s talking about then the alternator voltage is more suitable but they are still not really suitable for charging with a simple constant voltage, and have similar problems with being stored at full charge. There is no easy way you can simply terminate the charge at say 85% as mentioned above.

We have developed BMS (battery management systems) in the past for LiFePO4 and one prominent feature of them is that the individual cells go out of balance quite quickly i.e 3.0V on one and 3.4V on the next. Big problems start to arise pretty soon if you don’t balance the cells by bleeding current from the midpoints.

I just can’t see how you can run these reliably from a conventional alternator.
 
ph001 said:
Ok, if its LiFePO4 he’s talking about then the alternator voltage is more suitable but they are still not really suitable for charging with a simple constant voltage, and have similar problems with being stored at full charge. There is no easy way you can simply terminate the charge at say 85% as mentioned above.

We have developed BMS (battery management systems) in the past for LiFePO4 and one prominent feature of them is that the individual cells go out of balance quite quickly i.e 3.0V on one and 3.4V on the next. Big problems start to arise pretty soon if you don’t balance the cells by bleeding current from the midpoints.

I just can’t see how you can run these reliably from a conventional alternator.
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Sounds like you know more than I do, but I haven't heard of any issues from people running similar setups, it's quite common now. Anyway at £180 it's the cheapest 20kg I'll ever save so if I have to replace it at some point so be it.
 
TomK said:
Sounds like you know more than I do, but I haven't heard of any issues from people running similar setups
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I’m intrigued by this also. A few superbikes (and the McLaran MP4) run them oem but they have dedicated controllers (BMS) to control the charge and balance the cells properly. Maybe these are available aftermarket for cars? If people really are running them straight across a standard alternator it would be a big surprise to me that they get any kind of half decent lifetime out of them.
 
TomK, what exactly do you have installed on yours? All the further research I have done on this suggests you must run a separate charge controller (BMS) as I was saying above. There a few ‘smart’ batteries around that appear to have these integrated into the battery but they look very expensive.

This article explains it in quite simple terms... https://www.roadandtrack.com/new-cars/videos/a18350/lithium-batteries-not-just-for-hybrids/
 
Nothing, just as a replacement for the lead-acid as the manufacturer suggests. Their bumpf includes "maintenance-free due to intelligent BMS-System (over-/undervoltage, over-/undertemperatur, overcurrent & short ciruit)" which I guess is what you are talking about.
I see people using these types of batteries now all over the place, note the article you posted is from 2012.
 
fwiw... i used a small battery from a porsche gt3 a few years ago which i got cheap...... it lasted 6 months by the end of that summer wasnt holding charge, i assumed the car had overcharged it..... Ever since then ive stuck with the stock item. ( which weights alot :( )
 
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