DIY cranking or booster battery pack

[MikeT]

Pulling my hair out trying to find suppliers - so many seem to have gone bust or permanently out of stock.

Found an alternative to headway single cells - a readymade 4S2P 8.4Ah (40C burst) with balance lead.

[MikeT]

Hmm, that item can't be delivered (from EU warehouse) to the UK but it can from their Hongkong warehouse Go figure.

Meanwhile found a similar 4S1P 8Ah (40C burst) pack in their UK warehouse for £20 less so have ordered that instead.

[EDC5]

Great, looking forward to pictures when they arrive.

Have you considered coupling a supercapacitor to the lithium cells?

Some of those portable jump starters use supercaps and charge from the residual power left in a flat battery delivering it all at once. That way you could get away with a relatively small lithium capacity but with the capacity to deliver a huge cranking current.

[MikeT]

It's one of those prebuilt hobbyist zippy packs (made up of rectangular soft cells), bought from an online model shop. It's what I used originally as a portable car charger only this one has more voltage/current than the packs I was using. Comes with balance lead and XT90 connector.

I've seen those supercap booster packs - acts similar to a joule thief by utlising residual energy from a drained SLA. I've not researched it but could a small lithium provide as much energy as a drained 72Ah SLA can?

[EDC5]

I've seen those supercap booster packs - acts similar to a joule thief by utlising residual energy from a drained SLA. I've not researched it but could a small lithium provide as much energy as a drained 72Ah SLA can?

I think the main benefit would be the boost to the cranking amps, and it would prevent the lithium cells from being discharged too quickly.

[MikeT]

What's the purpose of the additional lithium cells in this scenario if the supercaps can draw sufficient energy from a depleted SLA?

[MikeT]

This is the LifePO4 pack I've ordered.

[MikeT]

Saw a video of a guy who made a capacitor pack to replace his SLA. He later added a zippy Lifepo4 4800mah 4S (to prevent discharge over time) which I noticed was swollen. He went on to say this damage to the Life pack was due to him using it (Life+caps) to jump start a ride-on mower. Without further detail, it's difficult to determine why the Life got damaged but I'm thinking the mower's depleted SLA was a discharge factor not taken into account?

I expect my pack to be capable of delivering 650A without damage though I'm bit short on real-life data of what Amps my car(s) use.

[white exec]

A clamp-on d.c. ammeter will confirm starter motor current.
If going for a small maintenance LiFePO₄ battery across a large cap, the battery needs to be dropped out when the large load is applied, as it will exceed the battery's max discharge capability. A current limiting resistor between batt and cap would allow the batt to maintain cap charge, but limit drain on the battery.

[EDC5]

That's true, perhaps a big resistor, on the order of a few hundred watts. That way the cap does the starting of the engine and is replenished by the LiFePO4, preventing the battery being without being over-current discharged.

Whilst the first few seconds of cranking draws about 300 amps, I suspect that the inrush to the starter could be much higher and it is that which can damage the LiFe, the caps should be fine with that though.

[white exec]

What I suggested above (the resistor) would only apply to using a smallish battery to keep a large capacitor topped up. Battery current delivery would only have to be sufficient to overcome the capacitor's self-discharge. So no need for a massive wattage resistor.
For a large battery and capacitor, disconnection of the battery probably ought to be by relay (and a suitable fuse!).
All significant recharging of the capacitor and battery would presumably by a mains charger.

[MikeT]

Not sure I want to fork out for a 600A ammeter (most are just 400A) only to find it reads OL during the coldest of cold starts.

Originally (last winter), test a portable maintenance charger idea for the C5. Unfortunately, I got too cold waiting and subsequently forgot about it but following the C3 battery going flat recently urged me to consider a booster pack, though I can still test both uses.

I may progress to trying capacitors at some time but the (SLA) budget is now spent on the Life cells. They seem a rather good alternative for stop/start systems I'd have thought.

As such, I was just reminded to dig out a power cube I bought many moons ago. The original gel battery had long since died and replaced it only last year, last charging it maybe six months back... It's now reading 10v and has given me the opportunity to further test the original idea - only I'm basking in the warmth this time

[white exec]

Can't see this data in the C5 pages, but for XM, where several levels of starter motor were fitted (2.5TD = Class 6) . . .

[GiveMeABreak]

The MK II C5 was only fitted with a starter from Class 3 to Class 6, depending on, engine type, equipment option and country of destination (for climate).

[MikeT]

Assuming I have the Class 6 500A starter, what would be worst case scenario - coldest day (-12c last year!), is it likely to exceed say the 600A limit of a clamp-on ammeter?

Alternatively, would Lexia be able to capture my cranking amps as Ellis recently posted from his or is that only provided by cars with battery monitors fitted?