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Backup Generator Charger (slrpnk.net)

submitted 1 week ago by oeuf@slrpnk.net to c/offgrid@slrpnk.net

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Can anyone recommend a charger (or type of charger) to use for recharging a 24v lead acid leisure battery bank from a backup generator? Currently we have a cheap charger bought online but it only puts out about 130 watts, which means running it for quite a long time to significantly recharge the batteries and is much less than gets fed to them when the sun shines. TIA

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[–] Nomad 3 points 1 week ago (2 children)

You need to determine the maximum charging power the battery bank can take so you can size the charge appropriately. Then take one that can take about 80% of the generator power and feed that into the battery bank. Size the charger to the lower of both numbers.

[–] perestroika@slrpnk.net 2 points 6 days ago* (last edited 6 days ago) (1 children)

Then take one that can take about 80% of the generator power and feed that into the battery bank. Size the charger to the lower of both numbers.

80% feels like too much.

My electric car has a 1600 W charging mode. My emergency gasoline generator supplies 2100 W theoretically if the power factor is 1 (totally resistive load). When I connect my charger in 1600 W mode, the generator trips and drops the load, however. It manages to work at 1200 W or below. And this happens despite the charger having a slow ramp-up behaviour (it doesn't go to full load in a second, but during 5 seconds).

So based on experience, I would advise to pick a charger that can take 50..60% of the generator's power.

For a 24V lead acid battery, look for chargers designed for warehouse equipment (forklifts), wheelchairs, truck batteries and such.

[–] Nomad 2 points 5 days ago (1 children)

Your car has a charge controller and a temperature sensor in the battery pack. A dumb charger with too much capacity could in theory overload the battery and get it too hot.

[–] perestroika@slrpnk.net 2 points 5 days ago* (last edited 5 days ago)

It definitely has. But in my case, I'm not bypassing it - the external part of the charger is communicating to the car using the Type 1 interface and telling "charge at X amps". (But I know that's not applicable to DIY situations.)

...and since the tripping occurs within the ramp-up period or very soon after (e.g. on second 7), I'm fairly certain nothing has managed to overheat yet. I think that some generators have poor steering regulators, mine included.

[–] oeuf@slrpnk.net 3 points 1 week ago (1 children)

That's great, thanks. And do you know if I should be looking for a generic automative charger, or something else?

[–] Nomad 1 points 1 week ago

Ideally you buy a BMS which is able to balance the charge in all cells of the bank. In terms of charging, the kind of charge applied depends heavily on the battery technology. Lead acid being the cheapest and easiest to charge. The BMS should be suited to that. Some are configurable to the kind of battery and capacity and number of cells. A car charger usually keeps one cell topped up. You need to keep them balanced so they are used equally and have similar voltage values.