Bad Batteries?

[Jim]

Need a little help from the battery gurus on the forum.

In preparation for my solar project, I bought 4-6 volt batteries, each 430ah. Put them in series/parallel and the voltage was showing just shy of 13 volts.

I had a 1,500-watt electric heater running in the MH and I disconnected the shore power. The inverter immediately took over. I went back and took a reading on the batteries and they were showing 11.4 volts and dropping.

Bad batteries?

 

[Neal]

Battery voltage can spike huge like this, it's normal IMHO. Try the same using your microwave. If your coach has a Magnum Inverter such as mine this is where the ME-BMK and ME-ARC50 are great additions so you base your numbers off of percentage "State of Charge" which doesn't spike like voltage. I use a lower LBCO (low battery cutoff) value, I think mine is 11.0 due to these spikes. Microwave or a high wattage item can hit your LBCO and shut you down in an instant.

When your high draw item stops look at where your voltage stabilizes. I'll defer to the battery experts here on how many batteries is enough and what your real net amp hour value is, 4 batteries is slim and can probably drain pretty fast. My coach for example has 8.

 

[Jim]

Not that I would run it this long, but my math says the batteries should carry a 1,500 watt load for 2.5 hours before hitting the 1/2 mark. Like I said, not that I would run that much of a load for that long, but mathematically, it seems like it should.

 

[Zeddy]

Think of batteries like an elastic band. Under load they will show falsely low and under charge they will read high. If you are using voltage as a state of charge indicator they need to settle with no load. When I use my coffee maker on the inverter they show 11.5-12volts but as soon as I turn off the coffee maker they pop up to around 12.5. That’s after powering my residential fridge all night.

 

[Jim]

Thank you @Zeddy. So the key here is to check the voltage AFTER the load off, and not while the load is on?

 

[Zeddy]

Yes, You need to consider the load if your using the voltage as a state of charge indicator. Newer coaches have so many things that are on all the time. If you have solar your batteries will look charged during the day and when the solar cuts out at dusk it will reveal how charged they are. It’s super confusing. I dry camping almost exclusively, I always run my generator in the evening after the solar has had a chance to charge all day if I need to.
Another thing to keep in mind is that many of these coaches do not have huge batter chargers. If your battery charger is say 50 amps you don’t get 50 amps of charge the whole time. Most chargers are three stage, meaning you get 50 amps of charge for the first(bulk) charger say up to 60-80% then it drops down until say 95% then trickles after that. If you are adding a huge bank you may want to consider upgrading your converter to one with a higher charge rate. One thing I found super handy was the Progressive Dynamics converters have an accessory called a charge wizard pendant. It’s a little box with an led which tells you what state your charger is at so you don’t waste time running your generator for hours to recoup that last 5 % of capacity. Some inverters also do this if they are inverter chargers.
Like Neal said above, the only thing you need to watch is that your voltage doesn’t go too low while running your inverter. That can damage your batteries and or your inverter. Refer to the chart above as a rough guide. If your voltage is too low on a regular basis you should consider more cells. The other thing that can be an issue is the size of your inverter wiring. If it is too small from your battery bank to the inverter it will also read low. Use the largest cable you can afford. As the voltage goes down current goes up so gauge comes into play.

 

[Jim]

Yes, You need to consider the load if your using the voltage as a state of charge indicator.

One thing I found super handy was the Progressive Dynamics converters have an accessory called a charge wizard pendant. It’s a little box with an led which tells you what state your charger is at so you don’t waste time running your generator for hours to recoup that last 5 % of capacity.

The other thing that can be an issue is the size of your inverter wiring. If it is too small from your battery bank to the inverter it will also read low. Use the largest cable you can afford.

I have a shunt and meter that I took off of my last motor home. It worked great and I always knew at what state of charge my batteries were. But I haven't installed it on this MH yet. Guess I better do that sooner than later.

As for the wiring from the battery bank to the inverter, that's not something that I installed. But my guess is that Tiffin used a ample size wire. One would hope anyway.

I just came back from the barn (where the MH is) and ran the heater and inverter for a couple of minutes. Then took a voltage reading on the battery. The voltage said 12.7 which is good, but the gauge on the solar controller showed the battery at 65%. So that's a little confusing but I'll get the shunt/meter installed and see what it says.

Thank you guys, the advice is always appreciated.

 

[Jim]

I hooked up the shunt/meter and did some testing. The readers digest version is as follows. I turned on the inverter and a couple of lights until I got a dependable 10 amp draw. This was at 1:30pm and the batteries read 12.79 volts. By 2:30 they were down to 12.40 and they stayed at 12.40 until I closed up the coach at 5:30. I would have let the experiment run all night but it's going to be 15* here tonight and I wanted the shore power and heat back on.

So I don't know. It seems strange to me that in the first hour the voltage dropped so much and then remained there for the next 3 hrs. I would have thought that a 30 amp drop would have made a change on the meter but maybe not.

I'll continue the test tomorrow.

 

[Jim]

Results of today's test:

My digital multimeter measures and displays Voltage/Current/Power/Energy simultaneously. Using this multimeter to monitor the battery bank, I put a constant 20amp draw on the batteries. The voltage started out at 12.80 and dropped to 12.24 in 4 hours.

The (430ah each) batteries are connected in series/parallel, giving me 860ah. Assuming I wouldn’t want to discharge below 50%, I would still have 430ah of usable storage. At 20 amps per hour discharge, they should last 21.5 hours (give or take a bit). At the current rate of discharge, these batteries will hit 50% in another 2 hours, which is effectively 29% of what they should be doing.

I'm pretty sure my math is correct, but feel free to fact-check it if you feel spunky. They may not be dead, but they're definitely damaged.

 

[Neal]

Please take a picture of one of your batteries so we can see what type and rating they are? 830ah for 4 batteries does not sound right.

 

[Jim]

Please take a picture of one of your batteries so we can see what type and rating they are? 830ah for 4 batteries does not sound right.

Data here.

 

[Neal]

Very interesting! Those are big batteries, mine are 225 Ah @ 20hr rated of which I have 8.

 

[Jim]

Yes, they are big boys. I had to modify the battery compartment to make them fit. Was going to install 6 of them but the weight was a concern.

 

[Jim]

Very interesting! Those are big batteries, mine are 225 Ah @ 20hr rated of which I have 8.

I assume yours are 6-volt configured in series/parallel as well? Or about 450 ah of usable storage?

 

[Neal]

Yes, 8 x 6V batteries each pair makes 12V so you take 225 x 4 for the total Ah of the battery bank.

 

[Jim]

Yes, 8 x 6V batteries each pair makes 12V so you take 225 x 4 for the total Ah of the battery bank.

Roger. Don't forget to divide by 2, don't want to discharge below 50%

 

[sheridany]

This is a really helpful thread as I suspect my batteries are in a similar state however I need to confirm as @Jim has done. I also assume I have the same battery bank as @Neal since our VTDP coaches are only a year apart. What’s the next step in this diagnosis or do you need just need to replace some or one based on the findings?

one question for @Jim so I can replicate, I assume you created the 20 amp draw from lights refrigerator micro other appliances being on? When you mention you are measuring the battery bank with your multi meter could you elaborate on that? At the main terminal or each battery pair etc?

 

[Jim]

one question for @Jim so I can replicate, I assume you created the 20 amp draw from lights refrigerator micro other appliances being on? When you mention you are measuring the battery bank with your multi meter could you elaborate on that? At the main terminal or each battery pair etc?

I connected the shunt and meter (shown in the picture below) to the battery bank. I bought this setup on Amazon but I suspect some of the higher-end coaches (like you're and Neal's) may already have something similar installed. I can't test each battery individually, well not without a lot of effort hooking/unhooking and moving them around, etc., and I'm not going to do that.

So yes, I turned the shore power off and then turned some lights on/off until I found the 20 amp draw I was looking for. (I suspect the inverter was 5-6 amps by itself.) I took a battery voltage reading off the meter, and then came back 4 hours later to take another reading. It' was just math from that point on although I'm going to go back down in a few minutes and get another reading as 4:45 will be the actual 6 hr mark.

I did this same test yesterday using a little over a 6 amp draw and I could watch the voltage dropping. Click here for a 60 second video.

 

[Neal]

What size is the shunt. I think the BMV-712 and ME-BMK are 500’s.

 

[Jim]

The ad says it's a 100A shunt. I'm afraid I don't know what the actually means.

Edit: I didn't necessarly mean that I don't understand the difference between 100 amps and 500 amps, just not sure if the shunt used in a 500 amp application would be more accurate than one that was used in a 100 amp application. (or visa versa)