Recharge a spare 12 volt SLA battery?

I found that a 12v 75amphour battery I have is at 50% charge. I was thinking I could go out and get a trickle charger and spend some money to get the battery up again.
But then I got to thinking that maybe I could do it for free if I used the leaf to recharge it.
Would the car's voltage regulator be enough to prevent an overcharge if I paralleled the 12v 75AH to the Leaf's accessory battery?
Or would I still need a separate charge controller in-between? Or because the batteries are different capacities that it won't work?

Thanks in advance.

foolios said:

I found that a 12v 75amphour battery I have is at 50% charge. I was thinking I could go out and get a trickle charger and spend some money to get the battery up again.
But then I got to thinking that maybe I could do it for free if I used the leaf to recharge it.
Would the car's voltage regulator be enough to prevent an overcharge if I paralleled the 12v 75AH to the Leaf's accessory battery?
Or would I still need a separate charge controller in-between? Or because the batteries are different capacities that it won't work?

Thanks in advance.

Click to expand...

the time to properly charge a 12v battery of that capacity runs in terms of days not just hours. You'll need a smart charger with a desulfinating cycle not just a dumb charger.

There is no reasonable way to get your car to charge the battery even if you were willing to tie it up and not drive it for several days.

I'm using the CTEK (56-158-1) MULTI US 3300 which will just barely do a 75ah battery. If I had a 75ah battery I'd step up to something beefier like the CTEK (56-353) MULTI US 7002 so it could charge twice as fast. But as long as you do it in a cool enough environment the cheaper one would eventually get the battery fully charged.

Using the Leaf or any other car on the planet to charge it would be slower than either of those options.

Thank you for the information.

If you are in no rush, I'd look at the Diehard 2 amp smart charger/maintainer. It could charge the 75AH battery in ~2 days and then maintain both that battery and the Leaf's.

LeftieBiker said:

If you are in no rush, I'd look at the Diehard 2 amp smart charger/maintainer. It could charge the 75AH battery in ~2 days and then maintain both that battery and the Leaf's.

Click to expand...

2 amp?

The Multi US 3300 is 3.3 amp and the Multi US 7002 is 7 amp.

Ive charged a Prius battery for more than 24 hours on 3.3 amp and it not been enough to fully recondition.

The only way a 2 amp charger is going to do it in less time is if it skips the conditioning phases and just brute force charges like a dumb charger.

I don't see that as a valid solution for a high AH battery and I'm not sure if it is AGM aware.

seriously take a few minutes and read http://www.amazon.com/CTEK-56-353-12-Volt-Battery-Charger/dp/B000FRLO9Y" onclick="window.open(this.href);return false; to see what all a truly smart charger does in the different cycles.

If by "conditioning" phase you mean de-sulfating, then you're looking at a more expensive charger than I thought was being sought. Sure a 7 amp smart charger (almost all chargers are "smart" these days) with a desulfating phase is the better choice, if the OP is willing to pay for one.

Thanks for the added info.

I had thought that the accessory battery in the leaf was charged by the DC-DC converter and main battery pack(480v) when the car was turned on. I can't remember if that's correct or if it's the wall charger for the car that takes care of topping off the accessory battery.
But what I was guessing at was that if I hooked up an external battery to the Leaf's accessory battery in parallel and then turned the car on, that it would raise the voltage in the spare battery rather quickly via the DC-DC converter and main battery pack(480v).

Thanks again.

It would try to charge the battery, but you'd risk both overcharging and undercharging, along with possible damage to the 12 volt charging circuit if it were left connected long enough to charge. If the 75AH battery is a sealed deep-cycle battery, you'd also risk 'cooking' it or even an explosion...

But even in that scenario with a charge controller in-between it'd still take days, right?
Or would a charge controller work between batteries, I'm guessing not, but I'll have to think about it.
Edit: Nope, I'm thinking it wouldn't. Have to have a higher charge to the controller...
Edit: But what if the connection to the accessory battery was paralleled to the spare via a charge controller and the vehicle was in Run mode?

At this point you're just being foolish. You can get a battery charger for $20-$30 that will charge and maintain that 75AH battery, spend twice that for one that will de-sulfate it as well, or you can spend the same amount trying to kill your car's charging system. Unless you want to actually install the big battery in the car, in which case it should be used to replace the OEM battery, not compete with it.

ya, I'm just playing with ideas. It's not worth hurting anything. just curious.

thanks all for the info.

foolios said:

I found that a 12v 75amphour battery I have is at 50% charge. I was thinking I could go out and get a trickle charger and spend some money to get the battery up again.
But then I got to thinking that maybe I could do it for free if I used the leaf to recharge it.
Would the car's voltage regulator be enough to prevent an overcharge if I paralleled the 12v 75AH to the Leaf's accessory battery?
Or would I still need a separate charge controller in-between? Or because the batteries are different capacities that it won't work?

Thanks in advance.

Click to expand...

Several problems. (1) the batteries are likely of different type -- flooded, gell, and sealed AGM have different voltages at charge, so will not charge evenly in parallel. (2) The small voltage drop from the paralleling cables can also make enough difference that the outside battery doesn't fully charge, even if they are the same type, age, and voltage.

Also, (3) many have noted that he leaf charger (from the 380 - 400 volt DC battery) doesn't provide high enough voltage for long enough to do a real good job of charging the lead-acid battery.

There are good, intellegent, maintainers for the various types of lead-acid batteries. You should use one of them.

I am. I bought one off Amazon.
THanks for all the helpful information everyone.