Any battery charging experts around?

Took the bus camping for three days over the weekend. Usually we have lots of 12 volt house power as I have installed three sets of trojan deep cycles that give me almost 700 amperes of reserve. But I managed to head up this time with dead batteries thinking that the 150 amp alternator would get them up to full charge on the trip. Wrong. I am learning as I go but I know now that the charge rate starts high and quickly drops as the voltage comes up on the batteries being charged. So the charge rate quickly dropped to around 50 amperes which meant that I arrived with a couple hundred amperes replaced. That got us through one day and then I had to run the generator. Its a big noisy diesel and I put up with it for a couple hours and the two 45 amp smart chargers got enough charge into the batteries to get us through another day. Did the same the third day and decided that I needed a way to get a fast charge if this happens again. So what would be the downside of taking along a shop charger with a 200 ampere start feature? Would there be a downside to having a full amperage charge for a couple hours to bring the batteries back to 80 or 90 per cent? Seems like a cheap simple solution. But not if it means boiling my expensive batteries to death.

fast charge usually equals massive heat problems. That leads to potentially explosive gasses building up around the batteries as well (definitely do not fast charge in an enclosed space near heat or spark).

The battery manufacturer should list specs on charging amps/times/acceptable temps/etc. Might be worth giving their tech department a call to see what they recommend.

But storing the batteries dead or nearly dead is really bad too. Get them on a 10 amp charger at home for a few days, and then leave them on a tender, or top up every couple months. Charging the batteries separately might be beneficial to charging them as a pack as well.

Also, your shop charger probably does not do 200 amps steady state. Its probably more like 30 seconds on time and 2 minutes off or something, with internal temp safeties built in.

If plugged into AC outlet, you need a full 20 amps at 120V to supply 200 amps at 12V, assuming no losses - which is an incorrect assumption. That is a TON of power.

Normally they are topped up and the bus plugged in. But it was inadvertently unplugged and I didn't notice until we left. No problem I thought as my big alternator will fix that. But I was wrong. I have lots of generator power to run a big charger but your point about the duty cycle is a good one so I will have to dig into that, The charger I have does not seem to cycle but maybe it does. Its only a 15 amp so I will have to throw my meter on there and see what it actually puts out.

The heating/boiling is not an issue as even at 200 amperes it would take over three hours to fully charge them.

Why not just idle the engine in that situation?

The only way you are going to keep the charge rate up is to keep raising the charger output voltage. If you get too high on the voltage you risk shorting out or reversing polarity of individual cells in the batteries.

In this case there is no way to speed up the process without a lot more specialized equipment and training. I have done fast charges on submarine main storage batteries but the equipment required is expensive and the amount of manpower to safely monitor the individual battery cells properly during the charge would make it impractical for a camper.

Kenny_McCormic wrote: Why not just idle the engine in that situation?

I actually did that for a bit but idling a Cat 3208 in the campground does not go over well! But the same problem applies anyway. Doesn't charge nearly quickly enough. The charge rate drops as the voltage comes up. So it's a very loud smokey 50 ampere charger at best.

In reply to bearmtnmartin:

Perhaps make up a small 12 volt generator out of a car alternator and small gas engine that is easily silenced? Like so https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/portable-man-cave-1972-13ft-boler-camper/65700/page4/

The primary problem you'll run into with a 200A starter is that they are only supposed to be used on 200A for a few minutes before they overheat.

What voltage are you seeing from bus charging system at the battery bank terminals? I'm guessing its low due to lots of long small gauge wiring with multiple high resistance connections. Decrease the resistance of the wires from the bus charging system to your battery bank and your voltage differential goes up and lets that big alternator use its amperage capacity.

I found out a long time ago that the vehicles alternator is not a good way to charge a dead battery. Too much of the output is being used for other things.

In reply to iceracer:

It's not that so much as the battery simply won't charge very fast at 13.8-14.2 volts. To the battery the alternator functions as a massively oversized float charger, as in normal use it's only supposed to top the cranking battery off after the engine starts, not fast charge a big deep cycle house battery bank.

This topic reminds me. I want to buy one of those solar powered battery chargers and see if they're any good for anything. I've been thinking about putting some batteries, probably deep cycle, in the race trailer to power things and not have to run that damn noisy generator all the time.

Kenny_McCormic wrote: In reply to iceracer: It's not that so much as the battery simply won't charge very fast at 13.8-14.2 volts. To the battery the alternator functions as a massively oversized float charger, as in normal use it's only supposed to top the cranking battery off after the engine starts, not fast charge a big deep cycle house battery bank.

It's even worse if you're a volt down at the battery terminals due to wiring resistance.

Battery capacity is measured in amp-hours not amps. Nitpicking I know, but amps and amp-hrs are not the same thing. Doesn't matter to your issue, but figure we may as well make sure we are talking with the correct terminology. Saying you have 700 amps in reserve in your battery is like saying that I have 100mph of gas in my tank.