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Author Topic: mixing batteries  (Read 284 times)

sunfighter

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mixing batteries
« on: September 17, 2017, 02:26:01 PM »
I'm about to add two 12v. 105ah ea. deep cycle batteries to my battery bank to get a little extra amp hours. (why you ask... because I have them and hate to see them go to waste)  ;) I already have four 6v. GC batteries in series parallel. I'm wondering if anyone has done it in an rv. Right now, I'm running 530 watts of solar through a sun saver duo 30 amp charge controller, and I feel this will be sufficient. Thoughts?
"Leaning in a heavy wind that no one else can feel"

2007 Gulf stream Independence triple slide. and 2005 Gulf Stream Independence triple slide

Gary RV_Wizard

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Re: mixing batteries
« Reply #1 on: September 17, 2017, 05:41:23 PM »
It's not an ideal situation and no doubt some will express horror at it. However, if the additional batteries are in good condition there is no practical reason not to do it. The issues arise if either the new batteries or the existing ones have much difference in their capacity and internal resistance. Since most people lack the knowledge and tools to really know that, the rule of thumb is "don't do it". Qualified techs can do it safely, but amateurs might best avoid trying.
« Last Edit: September 17, 2017, 05:42:56 PM by Gary RV_Wizard »
Gary
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Gary Brinck
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Home: Ocala National Forest, FL

AStravelers

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Re: mixing batteries
« Reply #2 on: September 17, 2017, 06:43:40 PM »
I'm about to add two 12v. 105ah ea. deep cycle batteries to my battery bank to get a little extra amp hours. (why you ask... because I have them and hate to see them go to waste)  ;) I already have four 6v. GC batteries in series parallel. I'm wondering if anyone has done it in an rv. Right now, I'm running 530 watts of solar through a sun saver duo 30 amp charge controller, and I feel this will be sufficient. Thoughts?
So you are going to mix two 105AH batteries with two 220AH batteries. Each pair of your 6V batteries makes a single 220AH 12V battery. I suspect you know that though.  Very interesting mix.   

My understanding of different capacity batteries in a battery bank is your capacity is limited to the weakest battery. 

With a pair of 105AH batteries matched with a pair of 220AH batteries, if/when your 105AH batteries voltage drops faster than the voltage on the 220AH batteries I believe your overall voltage will go down to the lowest battery thus negating the extra capacity.  Additionally you may find your charging may stop before your 220AH batteries are fully charged. 

If you do indeed add the two small batteries, keep a close eye on your charging/discharging and battery voltages.  It is an interesting experiment.  At some time when you have discharged your battery bank down to about 75% full (about 150AH's used) you might disconnect the batteries and see what the individual battery voltage is. 

Please come back and provide your results if you go through with this. 

Hopefully you have a battery monitor, like a Trimetric to closely monitor your batteries so you know for sure just what is happening. 
Al & Sharon
2006 Winnebago Sightseer 29R
2009 Chevy Colorado 4X4

http://downtheroadaroundthebend.blogspot.com/

AStravelers

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Re: mixing batteries
« Reply #3 on: September 17, 2017, 06:59:58 PM »
An additional thought.

Before going on a trip with this new setup take a 120V 1500 watt space heater and run it off your inverter for a few hours. 

The heater will pull a very constant 100 amps of 12VDC so you will know that each hour you have used 100AH.  After each hour of use you can turn off the inverter and measure the battery bank voltage and see how the voltage compares to the level of capacity. Use the battery voltage charts as related to the level of battery discharge to compare to the number of AH's you know you have used.  If your voltage is dropping faster than the number of AH's used show the voltage should drop, then you know this isn't working. 

Sorry that is confusing.  I hope you can make sense of it. 

Bottom line, using 150AH should bring your battery pack down to 75% full.  A 12V battery voltage chart lists a specific voltage for a battery 75% full.
Al & Sharon
2006 Winnebago Sightseer 29R
2009 Chevy Colorado 4X4

http://downtheroadaroundthebend.blogspot.com/

HueyPilotVN

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Re: mixing batteries
« Reply #4 on: September 17, 2017, 09:14:56 PM »
In my Stacker trailer I actually have four T-105 six volt batteries and two Optima Blue top 12 volt batteries.  I have 750 Watts of Solar and I hooked up two charge controllers to allow me have two seperate battery banks.  The Optimas power all the original 12 volt equipment and the T-105s are used for the chest freezer and to charge the golf cart thru an invertor.

Works for me.
« Last Edit: September 17, 2017, 09:29:23 PM by HueyPilotVN »
Bill Waugh
40' Country Coach DP
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35 years on the road

sunfighter

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Re: mixing batteries
« Reply #5 on: September 18, 2017, 08:19:01 AM »
Good answers and many thanks.  :)
 A little background. The 6v. batts are now 5 years old and are due to be replaced soon. When I do, I'm going to go to a 6 battery bank of 6v. However, I just wanted to experiment with the 12v. to see if this could work in the real world, and now seemed to be a good time. When I head out for my annual winter boondocking however, the coach will have a bank of 6 new 6v. batteries. Just wanted to be clear on that.
I like the idea of the 1500 watt heater as a way to measure the AH draw, and I do have a trimetric, hydrometer, multi-meter.

HueyPilotVN, for a while, I had a similar setup to yours. Oh, and thank you for your service. I was infantry and may have hitched a ride or two with you.  ;D 
"Leaning in a heavy wind that no one else can feel"

2007 Gulf stream Independence triple slide. and 2005 Gulf Stream Independence triple slide

Gary RV_Wizard

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Re: mixing batteries
« Reply #6 on: September 18, 2017, 08:46:43 AM »
Quote
With a pair of 105AH batteries matched with a pair of 220AH batteries, if/when your 105AH batteries voltage drops faster than the voltage on the 220AH batteries I believe your overall voltage will go down to the lowest battery thus negating the extra capacity.  Additionally you may find your charging may stop before your 220AH batteries are fully charged.

"Weakest" does not equate to "lowest AH capacity". It's more of a quality statement than quantity. Yes, the batteries will charge and discharge in different proportions, but the overall effect is small as long as all batteries are in good working condition.

With the batteries in parallel, the current flow will be in proportion to the voltage at each source. What usually occurs is that the larger AH capacity battery(s) decreases in voltage at a slower rate than the smaller ones and thus supplies a greater proportion of the total current. The proportions self-adjust so that the voltage in the system remains in balance.

The problems occur when one or more of the batteries have a different internal resistance, a situation that is hard to detect unless the difference is severe. It usually doesn't bet noticed until one of the batteries gets really hot. That makes the different battery scenario somewhat risky for those who tend to install batteries and then forget them.
« Last Edit: September 18, 2017, 08:48:14 AM by Gary RV_Wizard »
Gary
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John From Detroit

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Re: mixing batteries
« Reply #7 on: September 18, 2017, 09:43:06 AM »
I suspect those 105's are Group 29 MARINE/deep cycle.

Now. as for anticipated problems.. I suspect the phrase that apples is "Live long and Prosper"

I see no problem in what you are planing and I am doing.
Nothing adds excitement like something that is none of your business
My Home is where I park it.

sunfighter

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Re: mixing batteries
« Reply #8 on: September 18, 2017, 01:49:46 PM »
Detroit John,  bingo;
The batteries are super start marine deep cycle model 31DCM.  :)
"Leaning in a heavy wind that no one else can feel"

2007 Gulf stream Independence triple slide. and 2005 Gulf Stream Independence triple slide

AStravelers

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Re: mixing batteries
« Reply #9 on: September 18, 2017, 06:55:58 PM »
I suspect those 105's are Group 29 MARINE/deep cycle.

Now. as for anticipated problems.. I suspect the phrase that apples is "Live long and Prosper"

I see no problem in what you are planing and I am doing.
Does that mean you are mixing different sets of batteries, all in parallel, one set is twice the AH capacity of the other?  Anyways could you detail "what you are doing"?
Al & Sharon
2006 Winnebago Sightseer 29R
2009 Chevy Colorado 4X4

http://downtheroadaroundthebend.blogspot.com/

John From Detroit

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Re: mixing batteries
« Reply #10 on: September 19, 2017, 08:09:47 AM »
I have a pair of GC-2 230 amp hour DEKA Golf Car batteries in series

And in parallel with them 3 Group 29's.. You said the batteries you ahve are only 105 AH. the Group 31 is supposed to be closer to 130 (OH I used to have a pair of Group 31's but they are actually a tad hard to find).

All are also different ages spread over about 3 years

Hear are the fear monger myths, the facts, and the reasons.

First the reasons: For the batteries in Series is is important that they be twins, Both the same age, size, type, ideally consecutive serial numbers..  Mine are. (Well I don't know about the serial numbers withotu looking at the reiept)  as the fear mongers are right when it comes to batteries IN SERIES.

But in parallel
Fear: "Oh one battery will charge faster than the other and signal the converter "FULL UP" so the other never charges:
Fact: The State of charge determines the battery voltage As the battery charges voltages rise, the battery with the lower voltage (initally) will suck more of the charge current till they are in perfect lock step and so long as neither battery shorts or opens.... (Two failure modes) they will remain in perfect lock step for all time.  When one hits FULL the other does as well.

Fear: One will suck all the charging current and the other won't get charged.
Some Misdirected truth to this... The larger (GC-2 pair) WILL take more charge current, also provide more current to the load.. but the ratio will be EXACTLY the ratio of the batteries

That is if you put a 100 AH (rounded for ease) and a 200 AH battery in parallel the 200 will take 2/3 and provide 2/3 of the current.  So again they reach full or empty at precisely the same instant.

There is one concern but I've not seen it be an issue

THe GC-2's are designed for up to 50 percent discharge. the MARNE/deep cycles about half that (They want to be 75% full or more) but.. As I said, I did not see an issue there.

THE ONLY battery type Issue I ever had was when  tried AGM.. they were the newest batteries in the RV and the first to fail.
Nothing adds excitement like something that is none of your business
My Home is where I park it.

sunfighter

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Re: mixing batteries
« Reply #11 on: September 19, 2017, 08:29:45 AM »
Thanks big John, that's what I've been looking for, real world experience. The experiment will proceed.  :))
"Leaning in a heavy wind that no one else can feel"

2007 Gulf stream Independence triple slide. and 2005 Gulf Stream Independence triple slide

Gary RV_Wizard

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Re: mixing batteries
« Reply #12 on: September 19, 2017, 09:12:24 AM »
I ran a 12v AGM deep cycle in parallel with a series-pair of GC2 6v "golf car" batteries for nearly 5 years. The 6v's were only about 4 months old when I added the new 12v AGM, so it was a safe bet that all were is good condition. They charged off an Iota 90A 3-stage converter/charger.
Gary
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Home: Ocala National Forest, FL

AStravelers

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Re: mixing batteries
« Reply #13 on: September 19, 2017, 12:04:13 PM »
Good info.  I see where two knowledgeable people have hooked up miss matched batteries wired in parallel and they "worked". 

Just because it "works" does not provide a measured level of charge of the individual batteries when separated from the total battery pack.  All we know is that no apparent damage has been done. 

In the test I suggest below, the end result for all the batteries at the end of the discharge test and the charging test all the batteries should be at the same SOC "IF" mixing the batteries truly does not cause an imbalance of the SOC at the end of the discharge or charge period.  I realize there may be some minor differences because of the age differences of the batteries, but if one battery is at 50% or 100% and and the other lower or higher capacity battery is 10-20% different that would be significant. 

Doing this test would provide verifiable facts, not statements that it "works".

What I see is missing is a test I suggested earlier in this topic.  That is starting with your fully charged battery bank with a known AH capacity, then taking a constant AH drain such as a pure resistive of a 1500watt 120V space heater, discharging the battery to 50% of your known AH capacity and then disconnecting the batteries and measuring the voltages of the 12V batteries (i.e. including the 6V pairs) to see what the individual battery SOC is.  It would be good to measure the SOC of all the individual batteries before starting the test to verify they are all at 100%.  If not all the same, then charge them individually to 100%.

Additionally after bringing the batteries down to the 50% level the next test would be to charge the battery pack and when the total number of AH's put back in the pack indicate the batteries should be at the 95-100% SOC to then disconnect the batteries and see if the voltage truly shows all the batteries are at the 95-100% level. 

Since all on this discussion are very knowledgeable of lead acid batteries, I don't need to state that when measuring the individual batteries that you need to let the batteries rest the appropriate amount of time to get the true SOC. Not having the battery showning a surface charge after charging, and to allow the battery voltage to stabilize or recover after being discharged.

For those of us who dry camp/boondock for long periods and wanting long life from our batteries it is important to know from testing what the SOC charge is.
Al & Sharon
2006 Winnebago Sightseer 29R
2009 Chevy Colorado 4X4

http://downtheroadaroundthebend.blogspot.com/

kdbgoat

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Re: mixing batteries
« Reply #14 on: September 19, 2017, 12:09:29 PM »
The OP has two batteries essentially going to waste and a set of 5 year old batteries, and he's replacing all before winter. Sunfighter- hook 'em up and get all you can out of them before replacing them all.
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