Re-purposing a roller blind battery

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Tim+ 3 years ago

I have a couple of 12.6V lithium battery packs for powering roller blinds. (Solux BMEB.9800, 2,600mAh each).

I also have an EV whose 12V lead acid battery is possibly nearing the end of its life. Apparently the duty cycle of 12V lead acid batteries in EVs is not as conducive to long life as the “stress” they receive in an normal car.

Anyhow, the battery may or may not die unexpectedly in the near future and a lot of EV owners buy those small lithium jump starter packs. They don’t need to be high capacity, just to provide enough power to boot the system up.

Anyhow, before I rush out and buy something dodgy off eBay or Amazon of dubious quality and capacity, I got to wondering if I could use my one or two battery packs to do the same job.

I’m guessing they’d need a suitable diode to protect them when the onboard charger kicks in and maybe (just guessing) something to protect them against connecting them to something with a low internal resistance. It may be that they would likely have protection against a short already built in for safety reasons.

Is this a barking idea or worth a go (given that I currently don’t have any other use for these battery packs at present).

Of course the sensible thing would be to buy a new battery but over the years I’ve had many flat lead acid batteries that have soldiered on for years after a flattening episode so I’d like to see how long I can keep it going for.

Tim

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jon 3 years ago

First you would need to determine whether the internal resistance is low enough to supply maybe 200A current loading for a couple of seconds.

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Bob Eager 3 years ago

I doubt it will need 200A.

All it has to do is power the electronics, and close the safety contactors for the EV battery.

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Tim+ 3 years ago

The troll doesn’t understand EVs. I only really need to “trickle charge” it up to near 12V.

Tim

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Theo 3 years ago

In my Prius the 12V battery needs to provide about 30A to open the high voltage battery contactor, run the inverter coolant pump and charge the braking system. That's why you need a decent sized battery, not some AAs. The 12V system also powers things like blowers, lights, dashboard, radio, rear demist, etc. Those add up to a good few amps. Plus the alarm and central locking when it is off.

The other problem, which is more of a gen2 Prius-specific thing, is that the DC-DC converter that charges the 12V from the high voltage battery only runs at 4A. So you can run all the above stuff off the 12V but if you have a tiny 12V it'll get drained faster than the DC-DC can keep it topped up.

Other cars will vary in terms of how they use the 12V system, but being able to shift the main contactors is something the 12V battery needs to do in any car - a jumpstart pack can do that, but a few 18650s probably not. (A power tool battery might, just about).

I don't think your blind battery is going to run the central locking for very long either, in which case how to do get into the car?

(You would also need a BMS to manage charging and discharging, you can't just swap in some bare lithium cells without risk of them going explodey)

Theo

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Tim+ 3 years ago

No need. There’s a key entry option for when the battery is dead.

Well I’m not intending “swapping” a 12V lead acid for a teeny lithium battery, just using it to put enough charge in it to get the electronics booted.

Looks like it *should* be possible.

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Tim

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alan_m 3 years ago

I believe the starter packs and the battery packs you have are different beasts. One is designed for a lot of amps in a very short period and the other for continuous use at a lower current.

Bigclive

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Watch to the end so you don't make the same mistake :)

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Tim+ 3 years ago

Indeed, I never expected them to provide big amps. Thought they might be useful as a “battery powered trickle charger” over 15 minutes say.

Anyhow, I’ve splashed out on a new battery as the resting voltage of the old battery was struggling to reach 12.1V and sometimes dropping lower.

Tim

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Re-purposing a roller blind battery

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