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Milom · 10-08-2022, 01:19 PM

Hi all,
Just looking for an interim solution for my power needs, until new inverter arrives. MY Magnum inverter charger MS 2012 has stopped charging my battery bank since last April. So, all 110 plugs are dead as well as the res. fridge.
I thought I could just hook up a 40-amp portable charger to the battery bank and could charge them that way, but they won't get fully charged, 11-5- 12.0v max and as soon as i put a load on them, ie, turn the fridge back on, The inverter goes into low battery fault mode after just 2 hrs even with charger still connected? The Magnum has a 100-amp charger built in, but it is no longer functioning. How big a charger do y'all think I need? or is it even possible to do what i want with a portable charger?
TIA
Milo

wolfe10 · 10-08-2022, 01:23 PM

Unless you use a lot of DC power, a working 40 amp charger should be fine.

From your description of voltages, doesn't sound like it is working or the batteries are shot. If wet cell, have you checked battery water level?

What does your digital voltmeter say: before plugging in the charger? After plugging it on/turning it on?

Milom · 10-08-2022, 01:30 PM

Batteries were brand new last June, voltmeter says 11.0v at charge start and at 12 v the charger turns off?

pasdad1 · 10-08-2022, 01:40 PM

11v batteries are REALLY dead. How long did they sit discharged? One or more of the batteries may be toast. You should take pictures first of all the wire connections, then disconnect the batteries from one another and try to charge them separately. You may find some charge up to 13.6v or above just fine, and others will not. One might have a shorted cell, and it would just suck up all the power and get hot.

wolfe10 · 10-08-2022, 01:41 PM

Quote:

Originally Posted by Milom

Batteries were brand new last June, voltmeter says 11.0v at charge start and at 12 v the charger turns off?

Did you turn it off or did it turn off?

11 is DEAD battery
12 is "mostly dead" battery

And voltage when it is charging?
Amps when it is charging (if you know)?

What is going on with your solar charging (Four 160 watt panels)?

twinboat · 10-08-2022, 02:51 PM

Are you connecting the charger properly ?

Maybe send a picture of where your connecting the charger.

astrnmrtom · 10-09-2022, 11:47 PM

For a 12V battery - 10.5v is 0% charge, 11.3 is 10% so yes, they are for all intents and purposes, dead. Even 12v is around 45%. Since you are using 2-6v in series, even one bad battery in the pair will prevent the two from reaching full charge.

As mentioned above, disconnect the batteries and put them on the charger individually (or in pairs if you only have a 12v charger). If your charger has a high/low amperage switch, do the initial charge on high, and when the charger shuts off, switch it to low and let it charge for at least 24 hours. I had to do this if my batteries were drained down too far. The charger would shut off too early on high setting, and it took the lower setting to top them off.

One thing to note is measuring voltage right after charging isn't accurate. You are only measuring the surface charge and this will bleed off after about an hour which is why the batteries fall flat on their face when you put a load on them at 12v. If your batteries are deeply depleted, it can take 24 hours to bring them back to full charge. It's not uncommon for people to under estimate how long it takes to fully charge depleted batteries. It's because lead acid batteries, as the state of charge goes up, their ability to accept amps goes down. Even with a 40 amp charger, the final 10-20% could be only at 10amps or less.

Even with the charger connected, if the batteries are dead, they together with the inverter load are drawing more amperage than the charger can supply.

Some smart chargers can struggle with deeply depleted batteries. As a safety feature they won't charge a battery that doesn't respond normally. If you have access to an old style dumb charger try it.

40 amps should be plenty, the only thing a larger charger will do is shorten the charging time by being able to supply more amps early on.