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Old 08-05-2013, 09:37 AM   #1
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House batteries cause multiple glitches

When we bought our '05 National Dolphin class A in 2011, the pervious owner had replaced the 6V house batteries with two 12V deep cycle batteries.

Recently, one of the two gave up the electrons and this lead to multiple, intermittent problems: thermostat died, LP/AC refrigerator shutdown, and other installed appliances sometimes didn't work or didn't work completely as expected. It occurred to me that many of the RV appliance makers don't bother with including a 12v AC to 12V DC circuit in thier appliance and instead expect a good solid 12V DC from the house battery system.

What this means is that problems with supplying solid 12V DC can lead to all sorts of odd problems.

Now - and this is 20/20 hindsight - to accurately measure the voltage put out by each battery, YOU MUST REMOVE THEM FROM THE HOUSE CIRCUIT - AND FROM EACH OTHER (if you have a pair of them wired together). You don't have to physicall remove the batteries from their resting place if you can get to their posts, just remove the wires from the batter posts. (It is probably a very good idea to take enough pictures BEFORE you remove the wires, so that you can put them back correctly).

If the batteries don't have the expected voltage (12V or 6V) it is time to replace it or them and save yourself a lot of headaches (like the refrigerator full of food shutting off as you are going down the road - this tends to upset the DW .

J.J. Hayden (KN4SH)
Covington, GA
2005 National Dolphin 5342
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Old 08-05-2013, 12:06 PM   #2
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Yes, I agree that failing batteries can cause a lot of intermittent problems. I had that on the chassis circuit over nearly a year - just minor things like dash gauges playing up. Then couldn't get it to start.

Checked the batteries several times and couldn't really fault them, Checked all major ground connections etc etc. Long story short, eventually separated the batteries and put a heavy load on them and found one was giving up way too early.

New batteries and all the strange problems have disappeared.

The point? Yes there is one and it is that just checking batteries with a meter will usually show it OK (unless a cell is completely shorted or open circuit) so you need to be able to apply a load to them and make sure the terminal voltage is holding up while the current is flowing.

Tony Lee - International Grey Nomad. Picasa Album - Travel Map
RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper
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Old 08-06-2013, 05:05 PM   #3
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I keep reading how two 12's are better because if one fails....

Sure it works that way.. As you found out...NOT!

(Truth be told there is no TECHNICAL reason to prefer one over the other but there are other conerns.,
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Old 08-06-2013, 06:14 PM   #4
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Originally Posted by wa8yxm View Post
I keep reading how two 12's are better because if one fails....
Sure it works that way.. As you found out...NOT!

(Truth be told there is no TECHNICAL reason to prefer one over the other but there are other conerns.,
Well YXM,

First, I have to agree with you - íNOT!

Then- I do have some truth for you. I do work on very expensive boats and I charge a lot to do it. I have had to clean up after more than a few battery explosions. All but a very few were paralleled banks. I have only seen one single battery explosion in the last twenty years, but I used to get paid to clean up and repair the damage from for or five a season.

There are also internal resistance and drain that come to bite you in the stern with paralleled banks. If you can't charge each separately, the best you can get is about AH*1.5 from the 2 and that is only that good if they came off the line in the same minute and were never other that side by each until they were installed.

Sell the 2 ea GC2s rather than two of anything in parallel.

Trust me, it will work better.

A lifelong waterman and his bride going dry places for as long as the fuel money lasts.
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Old 08-07-2013, 07:26 AM   #5
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The irony is that after one of the batteries failed, I did take it out of the circuit and limped along on one 12v until I got to a place where I could get two new batteries.
J.J. Hayden (KN4SH)
Covington, GA
2005 National Dolphin 5342
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Old 08-07-2013, 12:52 PM   #6
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Just requires planning and care

In Telecom we have parallel plants everywhere to provide redundacy.

All batteries are connected to a single DC Plant that floats the batteries and powers the facility, zer need to isolate the batteries for charging unless you are performing load or other testing on isolated units.

I never have worked on marine systems so there may be marine specific rules or processes that are unique to them, however I would think their design criteria would be to avoid engineering that would lead to explosion of batteries.

And stating that you only get 1.5 capacity when you have 2 strings is not correct, unless you have something else going on.

You really get MORE than twice the capacity of each single battery due to the rating of the batteries, look for yourself by reviewing the data sheet for standby batteries, if the load per string is reduced the actual capacity of the battery is higher, so having two batteries reduces the load per battery resulting in a higher than thought capacity.

Battery capacity does not really go up, the installed capacity goes up.

What I mean is a single battery installed with a given load will have a set capacity that is different from the label, the label is usually at the 8, 10 or 20 hour rate and not how it is usually installed, that means divide the labeled capacity by the hour rating and the resulting number is the load in AMPS that the battery will support for that amount of time.

Say a 100 amp hour battery has in the fine print, "100 Ah @10Hr" that means the battery will support 10 amps for 10 hours, it will NOT support 20 amps for 5 hours.

So in a given example with a made up battery, the 100 AH battery (8 hour rating) is only 70 amp hour due to loading.

Adding another battery one first assumes 200 amp hours because 100 + 100 + 200, but the battery actually is only at 70 due to loading.

They thought the battery needed help because they assume it was 100 amp hours and should run their 50 amp inverter (input load) for 2 hours.

This example it would run a little over an hour and 20 minutes or so.

So 70 + 70 = 140, but when the second battery is added the load per battery is 1/2 what it was before, so the made up battery capacity may be 85 amp hour per battery resulting in 85 + 85 + 170 amp hours.

This is less than the assumed labeled capacity of 200 amp hours so one may think odd, but one must look at the correct data when determineing the actual output of the Battery Plant, this is part of the engineering that is required for these.

The data sheets are all over the web, just search the battery manufacturer or ask them for it.

Starting batteries usually do not have these data sheets, storage batteries for standby service does, however a sheet for say a 70 hour VRLA battery could be used to estimate the capacity of a group 24 starting battery only to see how the loading per battery effects the capacity, not the actual capacity.

Now when one of the batteries fails, then it gets interesting as an open battery simple is removed from the plant,, the load per battery changes and the output is based on that, if a cell shorts then it loads down the other battery.

But they never fail that way...

Under normal conditions a failing battery should not cause the companion battery to blow up unless the charging system is overcharging the system and does not have any limiting factors.

It is seldom that more than one cell in a battery would short, and they usually do not short to zero ohms so the current from the companion battery would not be excessive.

If one battery fails it can be replaced with same model, it is prefered to use same manufacturer so the basic characteristics of both batteries are close, however if the remaining battery is older then both should be replaced as whatever stresses caused the first one to fail were also expressed to the remaining battery which means it has some degradation resulting from those stresses and the capacity may be reduced to the point where it will effect the companion new battery, thus both should be replaced.

Current standards have breakers or disconnects on each string so if one goes bad that would cause this kind of problem the disconnect would open.

There may be cases where poor installation practices combined with unprotected charging systems provided the right combination of circumstances to have batteries explode, and still this can happen under perfect conditions, but given the better quality of batteries today combined with good installation practices and maintenance of the batteries there should be little to worry about.

Tony & Lori
1989 Country Coach Savannah SE
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batteries, house batteries

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