Charging house batteries

oltrunt · September 16, 2018, 07:10:13 PM

I'm pulling my hair out (singular, not plural anymore) over how to use the alternator on my bus to charge my house batteries----without cooking the start batteries. I have a Newmar 1-2-120 battery isolator but I can't use it because my alt requires a tickle from the battery to energize. I've contemplated simply adding a switch to direct power from the alt to the house batteries but I'm afraid that will cause my 110 amp alt to over charge the start battery. So now I'm thinking about a switch that will isolate the start batteries at the same time it engages the house batteries. I'd purser this idea by simply disconnecting the battery with the engine running to see if it kept running but I'm worried about blowing something out--like the computer. Any Ideas? Jack

Bill B /bus · September 16, 2018, 07:34:27 PM

A quick refresher on battery charging. The voltage applied to the battery for charging determines how much current can flow into the battery. What is the alternator voltage with the engine running? That is your charging voltage. "12VDC"
systems charge at 13.7 to 14.4 VDC. Double that for a "24VDC" system. The size of the alternator does not effect battery charging, except in a negative way of not supplying sufficient current for the correct charge. I charge with 50DN alternator, 250A at 28V. I have seen a charge at 150A that tailed off to about 80A within 30 or 40 seconds as the battery voltage increased.

BTW that fancy battery isolater causes the charging voltage to be anywhere from 0.5 to 1.0 volt low. Best is to have a relay, at least a 200A continuous rating(I prefer 400A).

Think about installing a Battery Monitor, I think Trimetric is the only one available. That will show you voltage, current, state of charge, amp hours positive and negative, cumulative amp hours, number of cycles. If you are using wet cell lead acid batteries then the hydrometer will become your friend.

Bill

luvrbus · September 16, 2018, 08:23:30 PM

Get a sensing isolator or a battery separator www.texasindustrialelectric.com can fix you up

bobofthenorth · September 16, 2018, 08:45:13 PM

A Xantrex auxiliary charger (used to be called an "Echo" charger) may work for you.

http://www.xantrex.com/power-products/power-accessories/auxiliary-battery-charger.aspx

Jim Blackwood · September 16, 2018, 09:04:53 PM

See what you guys think of this idea:

Most alternators have a diode pack for the rectifier. IDK, maybe these have individual diodes. They are a 3 phase full wave array, meaning for each of the 3 field windings (with ends paired) one diode will direct - to ground while one directs + to the battery, x3 = 6 diodes.

So, if you want to charge 2 battery banks you could just get an extra diode pack and connect them in parallel then run the output of one to each battery bank. (proper heat sinks and all of course) So much for the output side. Whichever bank has the lowest charge will get the most current. For the control side hook your sense wire to the bank with the largest loads and both will be charged to full voltage, or you could get fancy with it and build a circuit that will send the lower of the two voltages to the alternator. Might need a transistor or two for that one.

Or just install a battery switch from a boat with the sense wire at the output and manually switch over. 1,2 or Both.

Jim

oltrunt · September 17, 2018, 09:49:05 AM

Thanks for all the responses!
Brian, your refresher course was well stated. I had checked the voltage output and it is 14.4v. In fooling with the isolator I did observe the voltage drop you mentioned and sort of shined it on as part of the cost of doing business. It looked to me like even with the drop the voltage was still "within" the normal range--though it would make getting a full charge less likely in the short term. I had to laugh at myself over my post. I jumped in as if I'd been discussing all this with you all and assumed we'd discussed the fact that my charging system was up to snuff.

Bob, Xantrex does make quality products--though expensive. I went to their web site but I didn't find a device that could be powered by an alternator--did I just miss finding it? Right now I use a Progressive Dynamics 9245C three stage power converter to charge up my house batteries when I am on shore or genny power. Many people run their generators while driving but I'm a little skiterish about that hence the interest in using the alt.

Jim, I like your idea about the extra diode pack and will give it some study but in all honesty and in keeping with KISS, the three way switching idea most appeals to me. My question there is: is the sense wire you refer to the same as the exciter wire to the alternator?

Clifford, the sensing isolator you pointed out looks like a good option. I'll find a wiring schematic for it and look at it next to the charging schematic for my bus and see if I can tell if they will play nice together.

Jack

richard5933 · September 17, 2018, 10:10:55 AM

If you want simple and effective you should look at this:

https://www.sterling-power-usa.com/SterlingPower12volt-12volt-30ampbatterytobatterycharger.aspx

Also available in a 60-amp version.

We use one to charge our 12v house batteries from our 24v chassis alternator. Only works when the alternator is making power. The wonderful thing about this type of device is that it is a true multi-stage charger and it will provide a proper charge for your house batteries regardless of the situation on your chassis batteries. The chassis batteries will continue to function as before and will be charged with the chassis alternator/regulator.

We've got ours set up so that I have to flip a switch to make it work, but it can be wired so that whenever it senses the chassis alternator charging it will send a charge to the house batteries.

Jim Blackwood · September 17, 2018, 11:12:35 AM

I'm not sure Jack. I've not been into a bus alternator so I'm not certain how the voltage regulator is set up. Typically though the regulator will output a voltage/current signal to the rotor winding to control the level of output, I'm most familiar with that being referred to as the exciter. Whereas the regulator also has an input directly from the battery which tells it the battery voltage, usually referred to as the sense lead. The value of a dedicated wire for that is it results in higher charging voltages. You can tell what it is by where it goes and then apply the relevant bus conventions. (The regulator often also has a control input from the ignition which switches it on, often configured to light a fault lamp in case of a failure. This is not the sense lead.)

My only concern with this setup is, what happens when one bank is fully charged and the other is fully discharged? Say you are sensing from the fully discharged bank so you get full current from the alternator. The voltage differential means most of that charge would go to the bank that needs it, but without question some would go to the fully charged one. How much is a math problem, based on the relative voltages and resistances of the battery cables. That could be worked out and if the level was low enough it would be acceptable. I don't have that answer, at least not right now. It's been a long time since I worked with those formulas and I'm rusty. But it isn't a complex calculation, just simple math. It has to be set up right.

The downside of a manual boat switch is location, but more expensive remote switches do exist. Ideally you would control it from the driver's seat. But by that point why not just use the existing solutions? They are probably nearly as cheap, and proven.

Jim

Geoff · September 17, 2018, 12:08:18 PM

This question was posted several years ago and someone who seemed to know what he was talking about stated that if a mixed batch of batteries, some charged and some discharged; that the charged ones would not over charge but the weak ones would charge and all even out.
That is, if they were all good batteries to begin with. I know if you have a bad one it will boil out and you will smell it.

bobofthenorth · September 17, 2018, 01:10:09 PM

Quote from: oltrunt on September 17, 2018, 09:49:05 AM

Bob, Xantrex does make quality products--though expensive. I went to their web site but I didn't find a device that could be powered by an alternator--did I just miss finding it? Right now I use a Progressive Dynamics 9245C three stage power converter to charge up my house batteries when I am on shore or genny power. Many people run their generators while driving but I'm a little skiterish about that hence the interest in using the alt.

ICBW but my understanding is that the Echo charger doesn't care where the "surplus" charge comes from. I used one on my boat for 7 years to keep the genset battery charged. I never replaced that battery, never charged it other than through the Echo charge (no genset charging to its own battery) and literally just ignored that battery. In my defense it was in kind of an ignorant place so it was easy to ignore. As far as I know it charged both underway off the engine alternators and on shore/gen power but to be honest I never measured it so I can't be 100% sure of that claim. Considering how it was wired however there is no way it could tell if the battery charge was coming out of the inverter or from the engine alternators.

richard5933 · September 17, 2018, 01:52:45 PM

In addition to the battery-to-battery charger I posted about earlier, we also use a Xantrex Echo Charger to keep our generator start battery topped off. It can send up to 15 amps from one battery bank to another. The way the manual shows it being connected is to have the house battery bank as the main supply, with the 15-amp output going to the start batteries. In that configuration, whenever the house batteries are being charged up to 15 amps can be used by the start battery to charge.

I don't see any reason why you couldn't turn it around the other way to use the chassis batteries to charge the house batteries. However, you'd be limited to a 15-amp charge. If you really ran down your house batteries overnight and tried to use the Echo Charger to charge them, I'm not sure if 15 amps will do it.

There is one other big drawback to the Echo Charger - if the differential between the two battery banks is too great (house batteries really run down) the Echo Charger will not send a charge through. That's happened to us a few times when we had trouble starting the generator and ran down the generator start battery. The Echo Charger wouldn't send a charge through.

I finally installed a switch to bridge the generator start battery to the house batteries so that I can bring up the charge on the generator battery until the differential is smaller. I don't like to leave them connected during normal use though, as the generator start battery is the only way we'd be able to start the generator if the house batteries inadvertently were run down too far.

oltrunt · September 17, 2018, 02:51:19 PM

Thanks Richard. I queried the mfg as to why when the unit was set in SVEM mode in cold weather there was a chance of the input battery not being charged enough to start a vehicle. I also asked if the BB1230 model would work with my 110 amp alternator. I'll share what I find out.
Jack

richard5933 · September 17, 2018, 04:17:16 PM

Quote from: oltrunt on September 17, 2018, 02:51:19 PM
Thanks Richard. I queried the mfg as to why when the unit was set in SVEM mode in cold weather there was a chance of the input battery not being charged enough to start a vehicle. I also asked if the BB1230 model would work with my 110 amp alternator. I'll share what I find out.
Jack

I'm going to PM you the US contact I have for Sterling. Not sure if you were aware of the US office or if you were trying to contact them in the UK.

About the SVEM mode - the answer is quite simple. By default, the unit is set to send a charge through to the house batteries only when the voltage is high enough to indicate that the chassis alternator is charging the chassis batteries. Usually when operating on the road the engine RPMs are elevated and there is no problem. The SVEM (Stationary Vehicle Engagement Mode) sets a lower threshold for the unit to send a charge to the house batteries, allowing the house batteries to be charged even while the engine is idling. Depending on the function of your particular alternator at idle, it may not be providing adequate output to charge batteries. By forcing the unit to send a charge through, you run the risk of powering the charger from the chassis battery instead of from the chassis alternator. Do this for too long a time and the chassis batteries can be run down to the point they won't restart the engine in on a cold day. Guess they figure you're better warned than not.

I installed ours using the ignition input mode, but instead of connecting the lead to the 'ignition' circuit I connected it to a switched power source. That allows me to turn the battery-to-battery (BTB) charger on whenever I want, or turn it off just as easily. I can override the minimum voltage threshold if I want, or if I know the chassis batteries to be weak leave the unit off and allow the chassis alternator's full output to go to the chassis batteries.

I don't see any reason that unit would not work on your alternator, unless of course you don't have 30 amps to spare. You'll have to do the math to see what else is drawing power from the chassis system while you're running down the road. If you're running an inverter or other high-current devices you might have a problem. I'm running the 50DN on my bus, so the current draw (35 amp @ 24v outputting 70 amps @ 12v) isn't a concern. At least it hasn't been yet.

I'm actually surprised that more people aren't running BTB chargers like this. There is a slight efficiency loss, but almost any method other than the most simple will have some amount of that. Seems like the boating world has been doing this for a long time. I love the idea because it allowed me to keep my house and chassis systems totally separate and yet still charge one from the other.

ol713 · September 17, 2018, 05:00:42 PM

Quote from: luvrbus on September 16, 2018, 08:23:30 PM
Get a sensing isolator or a battery separator www.texasindustrialelectric.com can fix you up

HI;
Nice selection of relays. I have a battery isolator and always wanted to add a relay.
The relay could combine the house and start batteries when in a fix.
Thanks, Merle.

thomasinnv · September 18, 2018, 08:46:07 AM

If it were me i would go with a big boy relay and a bi-directional charging relay. When either bank is being charged (ie. Running the engine or being plugged into shore power) the relay will automatically engage and charge the other bank. You can buy one with or without an auxiliary start feature built in. (It's really easy to just wire in a manual switch yourself without that option.) The relay control will automatically engage and disengage at preset voltage levels to prevent accidental discharge of the wrong bank.