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

[doc74ormor]

What do I need to do to charge the coach batteries off the bus alternator? I'd like to be able to charge the battery bank while driving, but isolate the bus starter batteries from the coach batteries when camping.

[Roadrunner]

Buy a constant duty solenoid. Run a cable from the positive side of your start battery to the large input lug on the solenoid. Run another cable from the large output lug of the solenoid to the positive on the coach batteries. Run a 12 gauge wire from a power source that is hot when the ignition is on. Use an inline fuse. That way when your ignition is on the batteries will be connected and will charge. When you shut the ignition off the solenoid will disconnect the batteries. If the ignition switch shuts the power source off when it is in the cranking position I run another 12 gauge wire and another inline fuse from the positive on one of the batteries to a toggle switch and then to the small terminal on the solenoid. That way if your start battery is dead you can flip the switch on and give yourself a jump.

[porkchopsandwiches]

Sure Power 120 Amp Multi-Battery Isolator - (Part#: 12023A)

One of these would work, too.

[roach711]

Both of the above suggestions will work fine. The cheapest way to connect/disconnect is a simple battery disconnect switch but you do have to remember to disconnect the two banks when parked to avoid running down the starting bank.

BWD S5404 - Cut-Off Switch | O'Reilly Auto Parts

[bubb, the real one]

I put an isolator between my starter batteries and one of my house battery banks, to save money/time/effort I would not do it again, I would just put a battery selector switch and then manually switch between charging one or both batteries/banks.

[EastCoastCB]

I had some bad experiences with isolators. They never seemed to work very well, but then I suck at messing with electricity.

[porkchopsandwiches]

Question: with the simple setup of the big solenoid between the bus batteries and house batteries, say you have 4 big deep cycle Trojan batteries for your house bank. Say you have run these down to empty (12.1 volts or whatever it is) and start your bus, let it run for a minute, then flip the switch to close the solenoid so the house batteries connect to the bus batteries. Is there a way to figure out how many surge amps we are talking about with this? Like will flipping all those on at once cause a huge voltage drop in the system and screw things up? I have a 160 amp alternator and right now, with the inverter hooked to the bus batteries (I only use it while driving) if the 120 volt fridge plugged into the inverter starts up while the engine is idling, the idle will drop for a second while it is starting. Will it do something like this too when you suddenly add the load of a bunch of discharged batteries? Do they self-meter charging current or how does this work?

Thanks.

[Booyah45828]

Although a solenoid sounds like the way to go, I wouldn't do it that way. It'd work great if all the batteries were equally sized, equal health, and equal discharge. But when they aren't I've seen bad things happen(burned wires, trashed alternators, boiled batteries, etc). An isolator will allow one alternator to charge/discharge each battery separately at a controlled rate.

To do it right, get an isolator from a good manufacturer.

[Booyah45828]

Originally Posted by porkchopsandwiches

Question: with the simple setup of the big solenoid between the bus batteries and house batteries, say you have 4 big deep cycle Trojan batteries for your house bank. Say you have run these down to empty (12.1 volts or whatever it is) and start your bus, let it run for a minute, then flip the switch to close the solenoid so the house batteries connect to the bus batteries. Is there a way to figure out how many surge amps we are talking about with this? Like will flipping all those on at once cause a huge voltage drop in the system and screw things up?

Yes, it will cause the voltage for the entire system to drop the second you flip the switch. On an old mechanical engine, might not be that big of a deal, but on your electronic engine it wouldn't be good at all. That is why they have isolators.

An isolator will:
A) Let the chassis voltage remain at a steady voltage
B) Charge the chassis batteries at their own rate
C) Separate the chassis and house batteries, yet use one alternator to charge both.
D) Charge the house batteries at slow rate(which is what you want if they were discharged slowly)

12.1 house volts is discharged but not that low. Imagine if you left a load on all week (It will happen) and voltage was at 6. Dumping that dead of a battery bank on an alternator will certainly peak the alternator out(shortened life), drop the system voltage way too low (bad for the computer), cause the bus batteries to discharge(something you don't want), and it will probably kill the house batteries as well due to the 100+ amp charge rate you just put on them.

None of that is good. Use an isolator.

[juliol]

Originally Posted by Roadrunner

Buy a constant duty solenoid. Run a cable from the positive side of your start battery to the large input lug on the solenoid. Run another cable from the large output lug of the solenoid to the positive on the coach batteries. Run a 12 gauge wire from a power source that is hot when the ignition is on. Use an inline fuse. That way when your ignition is on the batteries will be connected and will charge. When you shut the ignition off the solenoid will disconnect the batteries. If the ignition switch shuts the power source off when it is in the cranking position I run another 12 gauge wire and another inline fuse from the positive on one of the batteries to a toggle switch and then to the small terminal on the solenoid. That way if your start battery is dead you can flip the switch on and give yourself a jump.

Roadrunner, thank you for this advise, this is a good set up, what if while the bus is in storage would like to charge batteries with a solar panel, but still have the same set up you suggested.

J

[bubb, the real one]

lets see, it depends upon how many batteries you want to charge from your alternator and how low they are discharged,
Most decent deep cycle batteries need to receive a charge of 14.4 to 14.8 volts at the post to fully charge them, the only way to know for sure which voltage is needed is to check the battery manufacturer,

My trojans require a 14.8v charge to top them off.

Your bus probably puts out about 14.2 volts which means it will never fully charge the batteries and this will decrease the useful life of the battery if the alternator is the main battery charger.

It is doubtful that any isolator will put a correct bulk/absorption/float charge on the battery even if the alternator was putting out enough voltage, which means you can not depend upon the bus alternator to be the only source of deep cycle battery charging.

If the alternator is putting out 14.2 volts then less than that is actually reaching the battery due to line loss, even worse for the deep cycle battery, starter batteries are rarely deeply discharged and they can handle the low voltage but deep cycle cant.

Someone mentioned 4 trojans but I dont think you have mentioned the number of house batteries you will use.

Either a decent battery charger plugged into a 120 source or a generator which is using a decent external battery charger will be needed. Even if your generator has a built in 12v charger they rarely do a good bulk/absorption/float charge.

[porkchopsandwiches]

Originally Posted by Booyah45828

Yes, it will cause the voltage for the entire system to drop the second you flip the switch. On an old mechanical engine, might not be that big of a deal, but on your electronic engine it wouldn't be good at all. That is why they have isolators.

An isolator will:
A) Let the chassis voltage remain at a steady voltage
B) Charge the chassis batteries at their own rate
C) Separate the chassis and house batteries, yet use one alternator to charge both.
D) Charge the house batteries at slow rate(which is what you want if they were discharged slowly)

12.1 house volts is discharged but not that low. Imagine if you left a load on all week (It will happen) and voltage was at 6. Dumping that dead of a battery bank on an alternator will certainly peak the alternator out(shortened life), drop the system voltage way too low (bad for the computer), cause the bus batteries to discharge(something you don't want), and it will probably kill the house batteries as well due to the 100+ amp charge rate you just put on them.

None of that is good. Use an isolator.

Alright that is what I figured. Thanks. So in terms of rated amps for the isolator (80, 100, 120, 160, etc) should I get one thats at the rated capacity for my alternator (160 amps) or based on the highest load the system will see?

Thanks!

[family wagon]

Originally Posted by Booyah45828

Yes, it will cause the voltage for the entire system to drop the second you flip the switch. On an old mechanical engine, might not be that big of a deal, but on your electronic engine it wouldn't be good at all. That is why they have isolators.

An isolator will:
A) Let the chassis voltage remain at a steady voltage
B) Charge the chassis batteries at their own rate
C) Separate the chassis and house batteries, yet use one alternator to charge both.
D) Charge the house batteries at slow rate(which is what you want if they were discharged slowly)

12.1 house volts is discharged but not that low. Imagine if you left a load on all week (It will happen) and voltage was at 6. Dumping that dead of a battery bank on an alternator will certainly peak the alternator out(shortened life), drop the system voltage way too low (bad for the computer), cause the bus batteries to discharge(something you don't want), and it will probably kill the house batteries as well due to the 100+ amp charge rate you just put on them.

None of that is good. Use an isolator.

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.

[family wagon]

Originally Posted by porkchopsandwiches

Alright that is what I figured. Thanks. So in terms of rated amps for the isolator (80, 100, 120, 160, etc) should I get one thats at the rated capacity for my alternator (160 amps) or based on the highest load the system will see?

Thanks!

If you do go the isolator route, it should be sized for the highest current it could have to carry -- the rated output of your alternator, at least. Higher capacity is extra insurance against overheating and damaging the isolator.

[family wagon]

Along the isolator vein: I did get a call back from a guy at Wirthco, makers of the Battery Doctor products. He explained that one big thing that differentiates their isolator from the traditional is that it's smaller, lighter, and doesn't get nearly so hot as the diode type do. There was a little bit of hand-waving about how it also protects the aux battery from overcharge. I think what he was trying to say is that their device allows power to flow to the aux battery until it's charged, and then it disconnects the aux battery to prevent over-charge. The aux battery has to be drawn down a bit before the isolator would turn on again. Like I said it was a little ambiguous.. but I think that's what he was telling me. Theirs also has a button you can push to connect the banks together in case the start battery were discharged too much and couldn't start the engine. Last point: he didn't say whether my earlier suggestion about sizing the traditional isolator larger than the alternator was right or wrong, but he did say it's fine to choose one of their isolators that is rated lower than the alternator output. I guess the electronic control inside self-limits to prevent damage.

[Tango]

Like a few other folks here, I admit I am a total dunce at all things electrical. The only thing I know for certain is that it's a bad idea to stick bobby pins into those little holes in the wall. (Learned that one the hard way.)

Meanwhile, and also like others here, I'd like to put together a system that will help relieve me from thinking about things electrical. Things like making sure that both the Start and the House batteries are kept at peak levels and there when I need them. I came across a doodad that from my lack of wisdom appears to address at least some concerns but would love hear some feedback from those who actually know about this sort of thing.

It is the...
Inteli-Power® 4000 Series

"Mighty Mini" Compact All-In-One System...
AC/DC Distribution Panel and Inteli-Power Converter
with built-in Charge Wizard®



From what I am reading (and the little I am grasping) it seems to fulfill the role of a combination A/C & D/C distribution panel...and...incorporates a charge controller that works on several levels. Maintainer, quick charge & desulphate. And it does all this while reading the charge state of the battery and adjusting to its' needs.



Great if it does what I think it does, but...is there a way to use this device to automatically switch back & forth between Start & House while on the road from the alternator? My Cummins is turning a 250 amp alternator so on any given highway run, I would think the Start battery would be back up in no time. And once it is up, if there is a way to get the power to automatically switch over to the House system...I think it might save on genny time. And given that the fridge/freezer will be ON and drawing from the House bank most of the time I'm driving...well, it just seems like a good thing. If possible.


Like I said, I don't know Jack about the electrical stuff, but, I am getting close to having to learn and start installing.


Also wondering about operating things straight from the alt through an inverter?/converter? thingie.



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

[dond]

Our BB was equipped with a WC lift. The lift has its own battery and has a isolator already. I just connected my house batt's. positive to the WCL batt. pos. and all is happy in chargeland.

\/\/\/\/\/\/\/---zap!

[taskswap]

I checked out the wiring diagram for this thing - such as it is. The unit itself is fine and is a nice way to set up a basic panel. I have a different brand but almost identical unit like this in my camper.

However, it is not designed to manage two batteries -- you'll still need to set up an isolator for that if you want that feature. There's a reference in here about "Alt Battery Pos" but it's for an external disconnect switch, not a second battery.

Note that while using an isolator your engine can charge your house battery, but while hooked up to A/C power this converter wouldn't charge your engine battery. For those wanting more control and easier wiring and not minding a bit of manual intervention (or wanting it) I've had REALLY good luck with the Blue Sea Systems HD-3003:

https://www.bluesea.com/products/300...witch_with_AFD

The list price is $121 but they sell for less than half that in actual retail outlets. I think I got mine for $65. This particular model has an "alternator field disconnect" which prevents a condition known as a load dump. If you shut off the battery switch while the alternator is running, the alternator windings have a phase lag before they stop generating power - but there's no load to sink it. This can drive a huge voltage spike into your electronics as the field collapses. An AFD will disconnect the alternator field slightly before cutting the actual load connection. This isn't something that should happen to you under normal circumstances - but it stops you from killing all your electronics if you DO do something stupid.

Using a switch like this you would have your engine battery wired to the engine as normal, and also to one input on this switch. You'd wire the other battery to the other input. No isolator required. Your engine would always use the engine battery for cranking, but if it was weak/low you could use the "1+2" position on this switch to help boost it from the house batteries. You could do the same thing while connected to A/C power or the engine was running to charge both batteries. While parked, you could switch to just the "2" setting to power your house load without draining your engine battery.'

This unit is designed to manage batteries for marine diesels. Properly installed it will last forever. I get nothing from this company - just a big fan of this product.

[bansil]

also just flipping on headlights/park/clearance lights before switching loads will give the surge a way to cope with it, by giving a "variable resistance load (lots of small heating elements to absorb any spikes)
I always do this before hooking up my vehicles to use for jump starting, parking lights can save alt. diodes and such

[family wagon]

That's an interesting way of turning the problem around: wire the alternator output, starter, and all the house and vehicle systems together to a single 12v bus, then use a switch to select which battery bank it all draws from and charges to. I think it's a good solution. Just don't forget to flip the switch back to the "house battery only" position after stopping the engine!

Originally Posted by taskswap

Using a switch like this you would have your engine battery wired to the engine as normal, and also to one input on this switch. You'd wire the other battery to the other input. No isolator required. Your engine would always use the engine battery for cranking, but if it was weak/low you could use the "1+2" position on this switch to help boost it from the house batteries. You could do the same thing while connected to A/C power or the engine was running to charge both batteries. While parked, you could switch to just the "2" setting to power your house load without draining your engine battery.'

That description is different than the one in the product's manual, but it's a good adaptation. It prevents a person trying to crank the engine off the house battery alone.

If a person doesn't mind getting her fingers dirty with a transistor, here's an automated approach to the above. It connects the starting battery to the alternator output only when the key is in the run position. All batteries are charged while the engine runs, house and vehicle loads are drawn from the alternator, and starter battery is protected from discharge by house loads. It could be modified slightly to provide jump-start mode.