How do I charge the coach batteries using the vehicle alternator?

[Booyah45828]

Originally Posted by family wagon

I'm sorry this ran so long, but I hope it's helpful..
I have an interesting mix of agreement, skepticism, and disagreement. First, agreement: sudden connection of a discharged battery bank is Less Ideal. Also, most alternators aren't rated for continuous operation at full power -- a 160 amp alternator can provide 160 amps, if it's being turned fast enough by the engine, but because of the internal heating that results the duty cycle must be kept short. Continuous operation should be kept much lower than the "rated" output.

Regarding the claims for the isolator.. Many (most?) isolators are literally a big diode potted in an aluminum heat sink. A diode alone does not do A, B, nor D. What it does do is prevent current flowing the other way -- it prevents the house battery from depleting the starting battery for example. As for charge rate, the charging current will divide into the batteries according to their respective levels of discharge. If the start battery is nearly full and the house batteries are discharged, most of the current will go to the house batteries. That would happen even if they were just wired directly together, however, just as Booyah said, without the isolator the charging house batteries actually could discharge the start battery if they want to pull more current than the alternator wants to provide. So B is true, kind of.

Other isolators are more difficult to assess. The Battery Doctor isolators appear to be more active than just a diode. I put a call in to them to ask questions and I'll post again if I get a response from them.

It's also correct that the house battery bank may not appreciate a 100-amp charge current (the rate at which the battery was discharged is not relevant to the charge rate, however). Trojan's FAQ for example suggests a rate of 10%-13% is ideal, or up to 20% if you're in a hurry, of the 20-hour capacity rating of the battery. So a 225 Ah battery might ideally be limited to 22.5-29 amps, or up to 45 amps if you're in a hurry. This happens to be the boat I'm in at present. I have a pair of Trojans, 225 Ah capacity, and charge them from a generator which I want to run as little as possible. That's how I ended up choosing that 40 amp Black & Decker charger Nat and I were writing about last week.

What I'm about to suggest is not the gold standard in energy efficiency by any stretch, and it is a fair bit of manual intervention.. I suggest skipping the alternator-to-house battery connection entirely. Get a good high-current mains charger like the Black & Decker instead of an isolator. When you need to charge where there are hookups, plug it in. When you need to charge on the go, run your inverter off the engine alternator and plug the charger into that. Some day when you own or borrow a generator, the charger can run off that too.

bubb's comments about fully charging the battery are good. An isolator, especially the diode type, will actually prevent the batteries from being fully charged. The Schottky diode type has a 0.3 volt drop minimum, and rectifier type diodes drop about 0.7 volts.

I could have sworn that's how the last isolator I messed with operated(maybe it wasn't an isolator , I don't know). All I know is that it was on a van chassis, it had one alternator, it limited charging amperage to the auxiliary battery(20 amps or so), but allowed full charge to the truck battery.

About battery charging, I was taught different. Does Trojan specify anything about auto batteries? I was always told that if it was discharged quickly(like starting a vehicle) It can be recharged quickly, but if it was discharged slowly then you want to recharge it slowly. I could be wrong though.

I like your idea about the inverter and separate battery charger, especially since it can be ran off both 12v and shore based power. Honestly that's probably the best way to do it, that way you also control the rate you charge the house batteries.

[roach711]

Tango - I have that Progressive Dynamics power panel in my bus and am very happy with it but it's primarily a house battery charger and not set up to switch between battery banks. I have my house and starting banks cabled together with a disconnect switch in between so I can charge either or both banks by the alternator or the power panel three stage charger but so far I've never connected the two banks.

In practice I charge the house bank with the power panel charger since we don't use much power on a weekend camp out.

Starting batteries are designed to put out a big surge of power to start the engine and to be quickly recharged by the alternator. Deep cycle batteries like a slow discharge and a slow, multi-stage recharge. For those reasons and the reasons listed by others I have always charged the two banks separately.

[family wagon]

Originally Posted by Tango

Any thoughts? (I already know the one about "Hire a professional")

I'm almost certain that there was a condition in the account sign-up process that we had to promise not to hire a professional because we're too cheap for that. Wasn't there?

That IP4000 looks like a really nice unit for a conventional trailer. Things could get a little more murky if an inverter or engine-driven battery charging are added to the mix, depending on how you want them to operate together.

How do "real" class A coaches solve this? Surely they have a nice solution.... or is it just "run the genny" and even those nice coaches aren't set up to charge house batteries off the propulsion engine?

I linked to some dc-dc converters at Powerstream a few days ago. I use one of theirs as a battery charger on an electric-hydraulic dump trailer. Trailer builders often wire these so the trailer charges from the truck through a self-resetting circuit breaker. It's an ugly kludge: when the electric motor is run to pump oil and raise the dump bed, the circuit breaker opens. When the trailer's battery is low, the circuit breaker opens and closes repeatedly giving the battery just little bursts of charge. I put one of the units from Powerstream on that trailer and the lifetime of the deep cycle battery improved significantly because it was getting a proper full charge every time it was towed, and calls from rental customers complaining the battery ran dead went to zero. I think those would be a great option for charging house batteries in the bus, too.

[porkchopsandwiches]

I have searched and searched and I still don't have a good idea of what I need to do for this. Basically, I am leaving for a 6 week trip in 10 days (yeah yeah I know I'm a procrastinator) and need to build an aux bank of 2-4 deep cycle batteries for it. The easiest and most efficient way in my mind would be to wire this bank up to the starting batteries with that Blue Sea manual battery disconnect switch between the two. Then connect the two banks and start the bus (or start the bus then connect them?). But I am still worried about current surge and draw on the alternator when these banks are connected. For a 230 AH battery, it says the surge current would be around 40 amps per battery (if 4 batteries, 2 in parallel and 2 in series) I think this would be 40 amps at 12 volts for 2 batteries, so 80 amps for a "fast charge?" But in normal battery charging circumstances, is it the battery's chemistry limiting charging current flow, or the charger? Because if I have 4 of these batteries that are empty, then connect them to the alternator, it will surge current and ruin the alternator and probably the batteries. Basically, the way I want to wire this up is the simple way, with just the alternator charging everything, but I am not sure if that is possible.

I am beginning to think about the idea someone suggested about having the 2 banks never be able to be connected, and put the Blue Sea - type bank switch on the inverter power line rather than the battery banks to change the power input source to the inverter, so the inverter is running off bus batteries and running the fridge, etc and the aux bank charger while driving, then when stationary on battery power, I switch from the bus batteries to the aux batteries and turn off the charger. When plugged into grid power, I have the aux battery charger on, which presumably will serve as a power supply to all the 12 volt appliances I have too?

I usually don't get too confused about electrical stuff, but how to set this up is baffling me. Thanks.

[family wagon]

Originally Posted by porkchopsandwiches

I am beginning to think about the idea someone suggested about having the 2 banks never be able to be connected, and put the Blue Sea - type bank switch on the inverter power line rather than the battery banks to change the power input source to the inverter, so the inverter is running off bus batteries and running the fridge, etc and the aux bank charger while driving, then when stationary on battery power, I switch from the bus batteries to the aux batteries and turn off the charger. When plugged into grid power, I have the aux battery charger on, which presumably will serve as a power supply to all the 12 volt appliances I have too?

Given the time constraint I encourage you to do it that way. It doesn't score any points for automation, but it's easy to assemble, easy to understand, and easy to diagnose or repair in case there's any problem. You have it all figured out; there aren't any missing links still to be solved.

Even if you accidentally leave the inverter running from the starting batteries one night and run them dead, you're only delayed and not sunk: hook up inverter and charger to charge the start battery from the aux, and you'd be able to start the bus in one to a few hours (assuming a beefy charger like that Black & Decker 40 amp). Oh... do bring along a volt meter, just in case!

When plugged into grid power yes the charger should be able to run your 12v loads and charge the battery too (albeit slower). It will help tremendously to unplug things from the inverter and put those directly on the grid power so the charger efforts can be applied to charging.

[porkchopsandwiches]

Alright thanks. But I am still not sure about the surge load, say if I am running off the aux bank overnight, run that down completely. Do I reconnect the two banks and then start the bus, and the alternator is just maxing out? Do I start on just the starting bank and then connect them? Either way it would be a big surge of current to the deep cycle batteries which isn't good for the batteries or the alternator (or the engine / trans computer).

I did find a DC-DC charge controller: Solar Converters EQ 12/24-50A, DC to DC Up/Down Converter

I think these are meant for solar systems to charge from the panels to the batteries. I could hook something like this up being powered from the starting batteries while it is running, to the aux bank, to regulate a max charge rate of say 50 amps (I think is what that device does). But then I would have to have a power source switch going to the inverter and the 12 volt appliance lines, and switch those to run off the starting battery loop while the bus is running too, so the aux bank can charge while driving.

So many different ways to do it!

[family wagon]

Originally Posted by porkchopsandwiches

Alright thanks. But I am still not sure about the surge load, say if I am running off the aux bank overnight, run that down completely. Do I reconnect the two banks and then start the bus, and the alternator is just maxing out? Do I start on just the starting bank and then connect them? Either way it would be a big surge of current to the deep cycle batteries which isn't good for the batteries or the alternator (or the engine / trans computer).

I didn't comment about the surge load etc in the first paragraph since I voted for the method of the second paragraph. But since you asked.. I'm not exactly sure what to advise. My gut feel is that it doesn't make a big difference whether the banks are joined before or after starting -- the big important thing is to not disconnect either battery while the alternator is dumping power into it because a high voltage spike could hit the other things that are still connected to the alternator. Instead, shut down the engine (or disconnect the alternator field; one of those Blue Sea switches does this) before moving the switch out of the 1+2 position. Not knowing the size or duty cycle rating of the alternator, nor the size of the battery banks, it's difficult to guess whether this semi-uncontrolled charging could be stressful to the alternator or batteries.

The charging current drawn by a battery depends on how dead it is. When it's very dead it can sink "a lot" (whatever that means) of current if the source is able, but if it's just a little discharged then it won't accept much current.

Originally Posted by porkchopsandwiches

I did find a DC-DC charge controller: Solar Converters EQ 12/24-50A, DC to DC Up/Down Converter

I think these are meant for solar systems to charge from the panels to the batteries. I could hook something like this up being powered from the starting batteries while it is running, to the aux bank, to regulate a max charge rate of say 50 amps (I think is what that device does). But then I would have to have a power source switch going to the inverter and the 12 volt appliance lines, and switch those to run off the starting battery loop while the bus is running too, so the aux bank can charge while driving.

You have the dc battery-to-battery theory right, but that particular device is for a different purpose. It's for the case where a person has a battery bank of two 12 volt batteries in series which they charge with a 24 volt charger, but they have some 12 volt loads hooked up across just one of the batteries. This causes a charge imbalance because the 24 volt charging input goes equally into both batteries, but one battery has extra load drawn out. Its charge state is perpetually lower than the other battery; it is always under-charged and/or the other battery becomes over-charged. The device you linked is like Robin Hood: it takes from the rich (battery) and gives to the poor until both are charged equally.

This device has done a great job of battery-to-battery charging for me, but at just 8 amps output, it would give only about 48 AH of charging during a 6 hour drive. Multiple can be connected in parallel to increase the output and I think I've seen higher-power versions of the same. The tech support guys there at Lund/Powerstream are nice and knowledgeable; give them a call and see if they can recommend anything for you. They're here in the Mountain time zone so it's not too late to call today.

[jazty]

It seems like the easiest and safest option would be an inexpensive PWM charge controller. The kind used for wind and solar. Note that charging a battery with a wind turbine is very similar to an engines alternator.
These devices can be sized to the battery bank and properly charge deep cycle batteries, unlike a straight alternator solution.

Has anyone actually tried this? I considered it, but with 600 watts of solar panels I haven't had any need.

[family wagon]

Originally Posted by jazty

It seems like the easiest and safest option would be an inexpensive PWM charge controller. The kind used for wind and solar. Note that charging a battery with a wind turbine is very similar to an engines alternator.
These devices can be sized to the battery bank and properly charge deep cycle batteries, unlike a straight alternator solution.

Has anyone actually tried this? I considered it, but with 600 watts of solar panels I haven't had any need.

I thought about that, too. They're definitely affordable and available. But in a quick survey of just one or two PWM controllers I couldn't find any spec for the drop-out voltage. In regulator integrated circuits that's the term for how much higher the input voltage must be above the output in order for the regulator to work (below the drop-out voltage a regulator doesn't work properly). I guess the term could apply to a PWM charge controller.. Anyway, if the PWM controller is fed from say a 14.4 volt alternator, I wonder what its output voltage will be..? Surely it would rise high enough to partially charge the battery but I wonder whether it will rise high enough for a full charge.