Hi, I am trying to create a way I can charge my lead acid RV batteries faster. The RV has a 45 amp "converter", which is just a constant voltage 13.8v power supply. With my interior lights on and connected to this power supply, it was charging my discharged 85ah battery at 10a. So it does not charge very quickly.

I was thinking that when I wanted to charge the batteries more quickly (for example when running the gas powered 120v generator for the sole purpose of recharging the batteries) I would parallel in a 20A 13.8v power supply (I happen to have one on the shelf).

Would there be any problems with this? Should I be sure to turn off the 2nd power supply once the battery reaches 13.8v? What if I didn't, would the battery draw any current?

I have a similar question if I were to upgrade the standard constant-voltage 45A converter with a "smart" multistage one. If I did that, I think it would be even more important to turn off the 2nd 13.8v 20A power supply, so the smart charger could do it's work?

Thanks for any help.




Another alternative would be to use a different voltage 2nd power supply. I have several old computer-server power supplies that can deliver 60A at 12v. I might be able to modify them to raise the voltage a little, sometimes to 12.5-13 or so. Would this work better?

 

it was charging my discharged 85ah battery at 10a

You do not want the charge current to be any more than that or you risk damaging the battery.
That's already above the nominal suggested maximum charge current for a lead-acid battery of C/10 (0.1C) or 8.5A for your 85Ah battery.
Lead-acid batteries do not like fast charging.

 

Must be a lot of interior lights to limit the battery charge from a 45 amp converter.

I think you could probably add as many more 13.8 volt supplies in parallel as you wanted to and the charge won't change because you have to raise the charge voltage to raise the charge rate.

The charge rate will change at any given time depending on the battery charge at that time, and 13.8 volts is only trickle rate at fully charged.

But, as crutschow has pointed out, you are probably over the limit anyway.

As to a smart charger, they will only charge at the maximum rate they are rated for and adjust their output to maintain that level.

 

Sounds like a need to upgrade my house batteries to lithium then. My goal is to be able to recharge quickly.

A related question for this smart group of folks - if I upgrade the house batteries to a drop in lithum battery (such as this), how do you think the Ford E-450 chassis charging system will be affected? I believe both battery systems (house & chassis) are charged together when the engine is running. Will the lithiums draw too much current and kill my alternator? It's a 130A model. I've read it can make about half that at idle.

I know they make DC to DC chargers I can put in-between but they are expensive and this is already a large project (budget wise and labor wise). I am considering installing a switch so I can manually engage/disengage the house battery from the Ford charging system.

Thanks.

 

if I upgrade the house batteries to a drop in lithum battery (such as this), how do you think the Ford E-450 chassis charging system will be affected? I believe both battery systems (house & chassis) are charged together when the engine is running. Will the lithiums draw too much current and kill my alternator? It's a 130A model. I've read it can make about half that at idle.

The max charge current of the Li battery is 120A with a standard charge current of 60A so not sure what will happen.
If the charge current is too high, you could add a small resistance (say 0.01Ω) in series with the battery to reduce the current.
This resistance could be just the resistance of a cable to the battery.

 

When paralleling power supplies the one with the highest voltage will normally supply the most current until it goes into current limit......or fails. Using a LIPO battery will require a different kind of charging system. Can you add more Lead Acid battery capacity? The battery charger/eliminator in the RV is probably a very simply design without much voltage or current regulation. Most of them are usually not more than a transformer, rectifier and maybe a capacitor. Most of them are designed to use the battery as a giant capacitor to help smooth out the DC voltage.

As previously mentioned, lead acid batteries should be charged with a constant voltage source. When the batteries are depleted they will take a higher current initially and then taper off to a trickle charge. If you change battery types, you will need to consider the differences in charging requirements.