Battery Sizing For Inverter

Beatenbo · December 06, 2010, 08:15:48 PM

Haven't posted in a while. Hello ....I have a MCI F3500 and I installed an inverter for running my fridge, TV , Sat, ect. while driving to keep from running a 12KW gen for this small stuff. I am not looking to boondock or run long periods of time with the coach not running. I installed a 24v inverter because the bus system is 24v. Of course 2 standard 8Ds in series. I added 2 optimums in series for the inverter.....Now the question...I have a 4 gauge cable run from bus batteries to keep the inverter batteries charged while driving. I have a disconnect to keep from discharging bus batteries while not running engine. Will the 2 different size batteries charge up fully or the smaller ones charge and fool the alternator not keeping the 8Ds charged or will they equalize.

Sean · December 06, 2010, 09:03:59 PM

Eventually, they will equalize. The extra batteries will not "fool" the alternator, but if you have a charger in your inverter and your generator is running, that charger can fool the alternator's regulator and it might shut the alternator down. This is not harmful, per se, as the alternator is not needed at that point, however it might mean your tach will drop to zero and/or your charger or "not gen" tell-tale light might come on. I find this to be enough of a nuisance that I don't let the inverter charge while under way.

I am concerned, however, about your use of #4 wire. If you have the standard 270-amp bus alternator, you need to have at least 00 (2/0 or two-ought) connecting the two banks.

-Sean
http://OurOdyssey.BlogSpot.com

bevans6 · December 07, 2010, 05:35:07 AM

I have wondered about the size of cable needed for charging. How do you calculate the actual current load in a charging situation? I have always considered that the current level while charging will either be the maximum that a low current source, like a typical 45 amp charger, can put out or if you assume a high current-potential source like a DN50, that the maximum current will be balanced by the batteries themselves in some way. I am thinking about the typical stock setup in a bus - 24 volts, two 8D batteries at say 250 amp hour rating, which should want to be charged at no more that 50 amps, how does the "self regulation" down to 50 amps work? Or does it?

Brian

Sean · December 07, 2010, 06:11:09 AM

Quote from: bevans6 on December 07, 2010, 05:35:07 AM
... How do you calculate the actual current load in a charging situation? ... the maximum current will be balanced by the batteries themselves in some way. ... two 8D batteries at say 250 amp hour rating, which should want to be charged at no more that 50 amps, how does the "self regulation" down to 50 amps work?

This is a complex subject.

First off, 250 amp-hours of batteries will not "want" to be charged at no more than 50 amps -- if they are seriously depleted, they will "want" to be charged at a much higher rate than that. However, you don't "want" them to charge at any more than 50 amps, so you need to have a way to limit the current to that amount.

Given a hypothetical infinite-current charge source, the amount of current that batteries will draw from that source is a function of the size of the batteries, the state-of-charge, and the voltage presented by the charge source. The higher the voltage, the greater the current will be.

The way current-limiting chargers work is to reduce the charge voltage until the current is within the appropriate limit. Unfortunately, with a constant-voltage charge source such as an alternator, the current will rise to the amount demanded by the batteries at that voltage and SOC.

However, and this is the important point here, the amount of current on the OP's interconnection cables can be much higher than the current demanded by the batteries. In fact, it can and will be the sum of the maximum charge current demanded by the house batteries at their greatest depletion, and the maximum current demanded by the inverter and anything else connected to those batteries. So the cables must be sized at least as large as the inverter capacity plus battery charging needs.

That said, prudence dictates that the cables actually be sized for the maximum available output of the alternator, because the possibility always exists that more load will be presented to the system. That load may be completely unintentional, such as a bad battery, or a high-impedance short to ground. You don't want the cables melting or starting a fire under these circumstances.

-Sean
http://OurOdyssey.BlogSpot.com

belfert · December 07, 2010, 06:20:17 AM

Since we're kinda on the subject of tying the alternator to the house bank, how does one verify that this is working and not causing issues?

What I did is used the A/C solenoid to control tying the two banks together. I ran a set of wires to the front of the bus to control the solenoid. The A/C solenoid only gets power when the engine is running. The disadvantage here is I can't tie the two banks together for a jump start. I started the bus and turned on the solenoid once, but I got some results I didn't like and turned it right back off. (I don't recall what I didn't like. Nothing was damaged.)

I have 6 large AGM batteries that are around 800AH. I'm not sure if they can handle the full charge rate.

robertglines1 · December 07, 2010, 06:29:00 AM

Sean, I have two 12 volt 1500 watt continuous inverters. I know this is not the preferred choice but I have them just setting. How many batteries and what kind do I have to have? I have a bunch we took out of our boat (not at my location so can't ck size) know they are gell. Bob

Joe Camper · December 07, 2010, 07:09:11 AM

Robert I would approach that by first asking how will they be charging and at what rate combined with what the typical loads will be, THEN habits like underway or boondocking?

If you are only mostly just using a TV and only doing it while driving 1 good battery would work but if you are running a fridge and want to get 8hr while boon-docking that is a different story.

I use a 12 volt 1200 watt inverter off my stock P-U battery for power. Ran a circular saw off of it but it probably would not have done it for long.

I guess it could be safe to assume the charge going into the bank is greater than the loads on the inverter are taking from it. So then you are left with how big the bank will be according to how you can comfortably use the accessories powered thru it.

1 example of mistakes (for lack of a better term) I can think of is a friend of mine he stores his bus inside with 50 amp service and never ever turns off anything!!

He has a large family and underway needs the gen for a/c and when camping either has gen or shore power. He has 8 8D batteries..for what? I could sneak 4 of them into mine he would never know the difference LOL

He has 2 4000W inverters that also sit down in the belly, usually practically idling.

robertglines1 · December 07, 2010, 07:30:02 AM

load on inverter :maybe some 120volt lighting and refer going down road when temps don't require generator use. we don't boondock on purpose (just our choice).prob 10 amp at most.I have space for extra alt on engine dedicated to house Bat..Have all parts to do this on hand so cost is not the question.just how many of what size? My last venture into inverters 3 buses ago was unpleasant.I used the cheap inverter from harbor freight and two 8D bat. blown internal fuses.and broken TV among results.

Joe Camper · December 07, 2010, 08:02:20 AM

Well since you are starting from scratch you could start with a smaller bank like 4 and leave room for expansion say up to 8. You want a sufficient enough capacity without unnessessarily overspending and actually camping will best answer that question for you.

bevans6 · December 07, 2010, 08:17:23 AM

Sean, I appreciate your answer. I kind of consider the state of charge of the batteries as a kind of CEMF, and the current that flows while charging is somehow related to the difference in voltage between the battery's internal voltage, the supply voltage, and some resistance that I don't know about, maybe the battery internal resistance. This part of the deal has always confused the heck out of me!

Anyway in my installation I chose to use an existing 0 gauge wire that was already part of the bus system, and I just fused it at an appropriate level for that wire size. So far it has been fine. I know that it limits the ultimate ability of the inverter to supply current somewhat (200 amp fuse so nominally 4800 watts surge ability on a 3000 watt inverter) but I have successfully started the air conditioner many times, and I also leave it bridged to the start batteries for hot starts (I disconnect for the first start of the day, and leave it connected thereafter). I used this table http://www.powerstream.com/Wire_Size.htm which puts the ampacity of 0 gauge at 245 amps in chassis installations.

Cheers, Brian

Sean · December 07, 2010, 08:27:10 AM

Quote from: belfert on December 07, 2010, 06:20:17 AM
Since we're kinda on the subject of tying the alternator to the house bank, how does one verify that this is working and not causing issues?

The way I do it is with a shunt and an ammeter on the intertie. I can tell exactly how much current is flowing in either direction when the bridge is closed.

Quote
What I did is used the A/C solenoid to control tying the two banks together. I ran a set of wires to the front of the bus to control the solenoid. The A/C solenoid only gets power when the engine is running. The disadvantage here is I can't tie the two banks together for a jump start.

I recommend activating the solenoid with the relay signal from the alternator, plus an additional momentary override for starting purposes.

Quote
I have 6 large AGM batteries that are around 800AH. I'm not sure if they can handle the full charge rate.

800ah of AGM can accept a 400 amp bulk charge rate, so you are probably not in any danger of exceeding it.

Quote from: robertglines1 on December 07, 2010, 06:29:00 AM
Sean, I have two 12 volt 1500 watt continuous inverters. I know this is not the preferred choice but I have them just setting. How many batteries and what kind do I have to have?

Joe did a great job with his answer. What I will add is that while on battery power, most folks can use at most 30% (for flooded) or 50% (for AGM) of their amp-hour capacity, since the "usual" cycle is to charge to 80% SOC and discharge to 50% SOC (for flooded) or 30% SOC (for AGM).

Add up all your 120 volt loads, in watts, then multiply by .11 to get amp-hours per hour at 12 volts. Now multiply by the amount of time you need to run, and that will tell you the number of battery amp-hours you will use. Divide that number by .3 to get the total number of amp-hours you need for flooded, or by .5 for AGM.

Example: You have an average load of 500 watts full time, between your fridge and everything else. At 12vdc, you will use 55 amp-hours every hour (500*0.11). If you want to be able to go overnight, say 10 hours, that would be 550 amp-hours needed. You'd need 1,834 amp-hours of flooded batteries, or 1,100 amp-hours of AGM batteries to run your inverters for the 10 hours.

By contrast, if you needed only one hour, say between parking and getting hooked up, then you'd only need 184 amp-hours flooded or 110 AGM.

As long as you have your engine alternator connected to the batteries while driving, it should have no trouble providing the 55 amps in this example.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

Sean · December 07, 2010, 08:35:07 AM

Quote from: bevans6 on December 07, 2010, 08:17:23 AM
... I chose to use an existing 0 gauge wire that was already part of the bus system, and I just fused it at an appropriate level for that wire size. ...

That's the other way to do it, of course, fuse the intertie at whatever current it can handle.

Battery, alternator, and starter connections do not have to be fused (the only such exemptions), and using one is always a judgment call. What I tell folks here is to make sure that the alternator always has a dedicated path to at least one set of batteries with no intervening fuses; you don't want to damage an expensive alternator by blowing a fuse. Then make sure that this one path has cables appropriately sized for the maximum current the alternator can produce.

-Sean
http://OurOdyssey.BlogSpot.com

Beatenbo · December 07, 2010, 05:36:37 PM

Thanks, lots of info..Sean I have an inverter without charger. I never run the gen and inverter at the same time any way. Wire gauge shouldn't be an issue in this case with the 04 gauge. My coach doesn't have coach air or heat with the big blowers my other MCIs had. Got Cummins ISC 350 that has a Leese-Neville belt driven atl. that puts out about 150 amps. Thinking on this I have one 8D house battery that stays charged with a 70amp converter for house lights and 12v satellite. I think I will take the 2 optimums in parallel for house and add another 8D in series for inverter and have equal size cranking and inverter and also have dual power if ever needed for starting if bus batts were ever a little weak. However I just replaced with 2 new Deka high cranking amps for that little toy Cummins.