Charging 3 different kinds of banks with 2 alternators.

Hi all, I have a 40' trojan with twin 6-71 DD, and there are 3 different battery setups. First two are the stock system: 8D batteries, one for each engine but the port one is also used as a house battery running water pumps, interior lighting etc. The 3rd bank is 450AH of golf cart 6v batteries, powering a 2kw inverter running a fridge, microwave, and various electrical outlets in the boat. The PO set up this last system to only charge when the boat is plugged in at the dock, no onboard charging capacity. I found that inadequate as I spend a lot of time on the hook, so I installed 600w of solar panels which keeps everything fresh except on the wet months of the year. But since we are motoring from anchorage to anchorage it seemed crazy to not be charging this last bank when the engines are running, so I disconnected the starboard engine alternator from the start battery and connected it to charge this bank.

These engines light up immediately so I wasn't too concerned about the starboard battery going flat, but of course I can only start the engine so many times before that battery will be toast. Often I have to use an emergency solenoid to also use that battery to start the port engine, as the port battery gets pulled down as we sit at anchor. Long story short, I want to be able to charge the starboard battery as well as the port battery and house bank, all three batteries with different needs, with two alternators.

Recently a battery isolator came into my hands and I set it up so that the port engine alternator will charge the starboard battery, but we won't draw from it when we're using the port battery at anchor.

But here's my concern: the port battery is constantly being drawn way down. The starboard battery gets drawn down only for a few seconds at startup. My fear is that while the alternator is pumping out all that juice for the port battery, the starboard battery is going to get boiled alive. Is this a valid concern with these batteries? What's my (inexpensive) alternative?

The battery bank-inverter setup sounds a bit marginal to me.....but since its exists and you just invested int he panels, I'd suspect there's little desire to ditch it...

First off, I'd an ACR (automatic charging relay) in place of the 'isolator' as you eliminate the isolator's inefficiencies....one approach would be to use the ACR and have one engine/alternator charge both starting batteries and the other the 450AH bank....

another thing to consider is adding another alternator to charge the 450AH bank. I'd also be inclined to have the 'house functions' live off the 450AH bank vs the starting battery.

Finally, if the batteries are good and the alternators/regulators are functioning correctly, there's little risk of 'boiling' - the bigger concern is one battery (bank) suffering from a non-optimal charging source at the expense of the other bank - especially if they are 'mixed technologies'.....

Thanks for the suggestions. I too would prefer having all house functions off the 450AH battery bank, but presumably due to the PO's advanced age, he installed the new bank at the other end of the boat from my DC power panel. A couple hundred dollars worth of cable, plus the absurdity of sending the charge power forward only to send it right back again. Not an efficient setup.

It might be possible but adding another alternator would be a real PITA, logistically speaking. Just getting to the one requires all the dexterity I can muster, never mind trying to cobble in another bracket system.

Can you explain to me how it's possible not to overcharge the starboard battery? The batteries are new BTW, but if I'm pumping 14V out of my alternator as it charges the depleted port battery, and that same voltage is on the mostly charged starboard battery, won't it cook it?

The boat has a built in 120v battery charger for when we are plugged in, and that doesn't have separate charge output for the two batteries either, even though the manufacturer knew that one battery would get drawn down a lot more than the other one.

Energy flows into a battery from the alternator.....most alternators (old factory designs) control the voltage (fixed) and allow the current delivered to vary. Say your two batteries are lead-acid (vs one of the newer designs)...the regulator will likely have a voltage set point ~ 14.3 VDC and the current flowing into a battery will vary based on its state of charge....a 'fully charged' battery won't accept much current.

Now if you have a lead acid battery on one side and say an AGM on the other, then the regulator's set point will only be 'ok' for one of them and the other will likely have issues down the road (this is a 'bad implementation or a bad design')....BTW, newer batttery designs have 'optimal' charging curves and typically need 'newer designed' regulators to produce best performance...

if you are concerned, then by all means, measure whats going on at each battery (voltage and current)....then you'll know what you have and what to expect.

A single output charger is like your alternator - multi-bank chargers are pretty cheap nowadays....that expense may make sense....

Hmmm, I thought that the current that will flow into a battery is a function of voltage - ramp up the voltage and the current will increase. Even a charged one? I thought that's how you overcharge a battery, putting too high a voltage on it for the state of charge. Which is why we charge a battery with a higher voltage which drops off as the battery charges. Am I mistaken about that?

a lot depends upon the capabilities of the charging equipment you use....if things are functioning correctly, the voltage should adjust based on temperature as well as current into the battery (based on level of charge). if the voltage get too high, then yes, you are very likely to "overcharge the battery". If you have ancient charging equipment, its very likely that neither the voltage or the current are monitored (or controlled) anywhere close to optimal. DEKA makes decent batteries and publishes lots of tech data on their current product line - it makes for an enlightening experience....

Got it. Thanks for your help.