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Author Topic: Adding new batteries to old batteries  (Read 1042 times)

Jey

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Adding new batteries to old batteries
« on: September 15, 2019, 10:15:39 AM »
Good morning everyone,


My TT battery was toast (came with the trailer)

So I started researching batteries in my budget that I could use with a solar setup soon. I came upon the renogy deep cycle agm

https://www.renogy.com/deep-cycle-agm-battery-12-volt-100ah/#trustspot-widget-wrapper


Long story short the fiancť ending up buying me one (or us I guess) which was unexpected but completely appreciated since our TT currently sits with no battery.

But now Iím curious. I know in the world of electronics itís not suggested to mix new and old batteries. I wonít have the money for a solar setup for probably 3 more months. I intended to run two of these batteries and eventually three.

Would there be an issue pairing newer (but same exact one)  batteries to the one I will already own?

Back2PA

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Re: Adding new batteries to old batteries
« Reply #1 on: September 15, 2019, 10:41:32 AM »
Assuming you take care of the new one, it will be no problem adding a second identical one in a few months
Scott
2014 Montana High Country 343RL (37')
2011 SD F-250 Crewcab LB 4x4, 6.2 Gas, 10K gross
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Matt_C

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Re: Adding new batteries to old batteries
« Reply #2 on: September 15, 2019, 11:51:39 AM »
I am going to upset you now, because I have been where you are going. 

If you parallel two multi-cell lead acid batteries that are the same rating, you will never get capacity X2.  Even if the jars came off the line with sequential serial numbers, it just won't happen.  You can maybe get 1.75, but more likely 1.5.  It does not matter what they are: LA, AGM or Jell.  The bank also has to be assembled so the distribution resistance is IDENTICAL.  Not just nearly, but within Milliohms.

But, if you insist, please put a big honkin fuse between them so that when a cell shorts in one of the jars, you do not get an explosion. 

I do not have an answer for what to do if you (like I) have limited resources.  If you possibly can, spring for a pair of GC2s now and pay the interest on a credit card.  It will possibly delay the solar addition, but it would be worth the interest.

By the by, I used to do specialty work on Performance Cruisers (read as retired racing sloops), and I dealt with this irregularly.  I used to love cleaning up after battery explosions because it was always a "bill the insurance company" for lots of time and clean up material.

Matt
A lifelong waterman with a trophy wife and a pair of mutts going places we cannot get by boat.

solarman

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Re: Adding new batteries to old batteries
« Reply #3 on: September 15, 2019, 05:41:32 PM »

please disregard that post above, batteries don't explode when you connect them in parallel, nor are the losses so great that it's vital they
are connected with leads balanced within milliohms. this is an RV not a spacecraft.


to give you a professional answer,

yes, you may connect different age batteries together, it's just not a preferred practice.
just be aware that aging reduces available capacity and lifespan, as as "set"
the net effect is to accelerate aging on the newest battery until it matches the oldest battery so to speak.
it is good practice of course to place a fuse between parallel sets just in case of a cell failure but this is not a common event.


i would suggest you wait until your funds can cover the cost of a better solution.
however, system cost is really dependent on your expectations of a system that you are
not able to size yet until further analysis..

as an example, let's say you need a minimal 200 watt system, that's really minimal case, but useful as a ballpark
cost estimate.

with 200 watts of panel, 3 sun hours and approx 400 watt/hrs of daily use
you will need approx 225 A/Hr of battery and a 15 Amp charge controller.

so..

1. Solar panel approx $175
2. Batteries ( two GC2 fla's aka golf cart battery ) $200
3. charge controller. Victron 75/15 = $80
4. cabling, say $25
5. fusing, say $30

estimate is : $510 excluding labor and mounting hardware.

here is a link to my post if you need a more info.
http://www.rvforum.net/SMF_forum/index.php/topic,113514.0.html



@Matt_C..

the reason you have experienced batteries "exploding" is because of poorly designed systems that cannot tolerate single or
multiple failure events. . there is almost no case for this is a correctly designed system.

looks like you made a business cleaning up bad designs the hard way, there is nothing to love about
sulfuric acid cleanups in my mind. !
KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

SeilerBird

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Re: Adding new batteries to old batteries
« Reply #4 on: September 15, 2019, 06:14:04 PM »
Thank you solarman. ^^^^

Gary RV_Wizard

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Re: Adding new batteries to old batteries
« Reply #5 on: September 15, 2019, 06:18:30 PM »
All I can say is that my experience is nothing at all like Matt C's.  Especially with regard to brand new batteries in parallel, of which I've installed more than a few sets.   Maybe racing boat batteries short & explode on a regular basis, but it's a rare occurrence in RVs. For that matter, I've never had my fishing boat batteries explode either.


When combining  mixed age/condition/type batteries, you run the risk of decreased performance (the 1.5x factor Matt C mentioned). an age difference of 3 months with same type & size batteries is a very minimal risk.  Trouble is, in real life that tends to get stretched to maybe 6-12 months  and "almost" alike.  The further from the ideal you get, the greater the risk of sub-optimal results.
Gary
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Gary Brinck
Summers: Black Mountain, NC
Home: Ocala National Forest, FL

Mark_K5LXP

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Re: Adding new batteries to old batteries
« Reply #6 on: September 15, 2019, 10:32:25 PM »
A shorted cell isn't an automatic explosion.  Even a parallel bank where a good battery dumps into a mated battery with a shorted cell will only result in the bad battery getting a healthy charge current, which may result in cells eventually venting - not exploding.  I have personally had batteries suffer fault currents of hundreds of amps and the only result was steam coming out of the vent caps.

Fusing sounds like a good idea but practically speaking there wouldn't be much distinction between the current flow you'd see between a good and bad battery and normal discharge currents.  So a fuse big enough for normal operation probably wouldn't blow.  Consider that a discharging good 12V battery is less than 12.78V, and a 5-cell battery across it will reach full charge (2.5VPC) at 12.5V.  So the bad battery will be basically in equalize mode and drawing relatively very little current.  In use, this battery will be providing little to no useful amp hours, and charging of the good one will never complete because the absorption voltage will never be reached so you're not using a bank in this condition for very long.  Cells can short spontaneously but generally it's a gradual process, where a cell experiences diminished capacity and eventually is not able to be fully charged and no longer is contributing much to delivered Ah.  Sometimes confused with a "short" but it's really just "dead".  The way to tell them apart is to apply a long equalize charge.  A weak/dead cell will gas and likely be warmer than the others, a shorted cell will be cold and not gas.

Previous posts about the OP's question are spot on.  Under controlled conditions you might see an issue with delivered amp hours over the cycle life of a mismatched set vs a matched one.  RV's are anything but a controlled environment and your batteries are going to suffer any number of "insults" over the time you run them.  Batteries are consumables, like tires and oil they don't last forever and next time around after you've worked out the kinks with your solar system you can get a new set.  Maybe it will make a difference, maybe it won't.  Bottom line is do they run your stuff when you want.  That's the only reason you have them.

Mark B.
Albuquerque, NM

John From Detroit

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Re: Adding new batteries to old batteries
« Reply #7 on: September 16, 2019, 06:54:32 AM »
I have in the past posted details as to why you can mix different age or different size in parallel (When you put say a pair of GC-2 in series they need to be identical but in parallel it don't matter)

Most of the "Dire warnings" are like political promises. and as I often tell politicians I grew up on a dairy farm.. "What does that have to do with the issues"  I shoveled that stuff too.
Nothing adds excitement like something that is none of your business
My Home is where I park it.

Gary RV_Wizard

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Re: Adding new batteries to old batteries
« Reply #8 on: September 16, 2019, 10:29:41 AM »
Excellent and very clear explanation, Mark. 
Gary
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Gary Brinck
Summers: Black Mountain, NC
Home: Ocala National Forest, FL

Matt_C

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Re: Adding new batteries to old batteries
« Reply #9 on: September 16, 2019, 11:03:40 AM »
Solarman,

If the "Bad Designs" were the issue, then all of the books are wrong.  Three of the battery explosions I got paid to clean up were done by those books.  For two of them, I had put in the additional jar a season or two before.  I do not recall seeing a battery explosion when new batteries were properly parallel but such has been reported to me.

As to the loss of capacity, how often do you "Ring" (a test to measure the discharge and recharge slope) of any bank?  This is a very reliable way to measure the capacity of any bank.  To do this requires an accurate meter, an actual current measurement (I used a portable shunt before I got the Bell probe) and a small time-base data acquisition system, it is possible to determine the true bank capacity in a test that can be completed in an half of an hour.  In my case, this was a pair of RS232 analog inputs to a laptop.

Admittedly, the published capacity of 1.5 was a pair of Grp27 that were different, but one owner had a strapped twin pair of Grp31 AGMs and they first measured at ~1.76.  When I rearranged the feed points to be more equitable, we were able to get to 1.92.  That is still short of 2. 

If you want to know more about this, there is a good article the old Family Motorcoach (now Family RVing) https://familyrvingmag.com/2012/01/01/the-balance-of-power/.  It is also available as a *.pdf that has the pictures available.   

Matt
A lifelong waterman with a trophy wife and a pair of mutts going places we cannot get by boat.

solarman

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Re: Adding new batteries to old batteries
« Reply #10 on: September 16, 2019, 09:32:31 PM »
Solarman,

If the "Bad Designs" were the issue, then all of the books are wrong.  Three of the battery explosions I got paid to clean up were done by those books.  For two of them, I had put in the additional jar a season or two before.  I do not recall seeing a battery explosion when new batteries were properly parallel but such has been reported to me.

As to the loss of capacity, how often do you "Ring" (a test to measure the discharge and recharge slope) of any bank?  This is a very reliable way to measure the capacity of any bank.  To do this requires an accurate meter, an actual current measurement (I used a portable shunt before I got the Bell probe) and a small time-base data acquisition system, it is possible to determine the true bank capacity in a test that can be completed in an half of an hour.  In my case, this was a pair of RS232 analog inputs to a laptop.

Admittedly, the published capacity of 1.5 was a pair of Grp27 that were different, but one owner had a strapped twin pair of Grp31 AGMs and they first measured at ~1.76.  When I rearranged the feed points to be more equitable, we were able to get to 1.92.  That is still short of 2. 

If you want to know more about this, there is a good article the old Family Motorcoach (now Family RVing) https://familyrvingmag.com/2012/01/01/the-balance-of-power/.  It is also available as a *.pdf that has the pictures available.   

Matt


Bad designs..

not all books are wrong, just some and the users interpretation of the content..  it doesn't take much to effort to assemble a bad and dangerous design,
just take a look at any RV on the market.. !  I just shake my head in disbelief looking at some of the multiple parallel battery setups in these rigs.
sadly, the RV industry has yet to grow up and take advantage of high voltage low current systems.

Battery balance.

always an issue with parallel strings.. there are connection schemes that minimize imbalance, but it's always there as a function
of battery internal resistance so it's best avoided.. that's why 2 Volt cells exist !

if you need a 12 Volt 600 Ah battery then use 2 V 600 Ah cells.. there is almost no need to parallel batteries to meet a capacity goal.
there are cost and size constraints though, size, and to a lesser extent, weight, being the Achilles heel for an RV.

Capacity estimates..

I made it standard practice on all off grid designs I design, to fit a small SCADA system to monitor cell voltages and current flow.

I apologize in advance for sounding arrogant, but i don't need to know "more about that"..  I could have written that stuff 30 years ago..
all it does is highlight the need to transition away from low voltage ( especially parallel ) systems. there are many advantages moving
up to 24 or 48 V.



KZ MXT20 480 W solar
ORV 24RKS 960 Watts solar
48V LFP, 2000W inverter/charger
Ram 2500 CTD

Matt_C

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Re: Adding new batteries to old batteries
« Reply #11 on: September 17, 2019, 10:34:10 AM »
Solar,

Your last confirms that we agree much more than we disagree. 

With both smaller coaches and sailboats, weight is an important issue.  I am watching for LiFePO to get more price competitive. 

The higher system voltages are starting to show up in passcar systems, so the rest can't be too far behind.  One big problem I ran into on boats was that there was very little hardware available that was made to be native at even 24VDC.  Some trucks have been 24 for a while, but it is still tough to install a good stereo.

I have seen the stacked cells banks, but they are sometimes difficult to locate in the hull. 

Thanks

Matt
A lifelong waterman with a trophy wife and a pair of mutts going places we cannot get by boat.