Read Before Adding/Replacing Battery to an existing set, charging/performance issues

[Mustang65]

FYI

While researching battery maintenance/upgrades I came across this information that clears the air on adding/replacing/charging battery performance issues. Here is what was said regarding Parallel/Series and adding/replacing charging batteries. It is my understanding that the information came from Concorde Battery Company (Mfr of Military batteries). Very interesting information...

1. A new battery can be added in parallel with two (or more) older batteries. However, new batteries should never be connected in a series circuit with older batteries.


I then e-mailed them this question:

2. Why new and old batteries should never be connected in series?
-from the Director of Advanced Battery Technology at Concorde
The reason for this rule is because new batteries have a lower rate of oxygen recombination compared to aged batteries. When connected in series, the same current goes through each battery. The voltage of the new battery will get too high because of the lower oxygen recombination rate. The high voltage will force the new battery to be overcharged, leading to accelerated grid corrosion and loss of electrolyte. The high voltage on the new battery will also mean a lower than normal charging voltage on the remaining aged batteries so they will get undercharged, leading to sulfation and loss of capacity.



- from their Vice President of Manufacturing:
· In a series circuit, the capacity (A/H) input (charged) or removed (discharged) stays constant throughout the entire string. Therefore, if 50 A/H is removed from the string, it will be removed from every battery within that string.

· The measured voltage of a string is the sum of the voltages of the entire string. By the way, the current removed is not cumulative.

· Therefore, if you have a series string with several batteries and one is lower capacity than the rest of the string, the net voltage of the string will be supported mostly by the “new” batteries and may allow the string to be discharged below the capacity of the lower capacity battery.

· If this occurs and the discharge continues, the lower capacity battery will become completely discharged. Once the voltage of this battery reaches zero and the discharge (driven by the rest of the batteries in the string) continues, the current which is discharging the rest of the string will also travel in the discharge direction through the lower capacity battery, in effect charging it in reverse. This will destroy the lower capacity battery.


· In a parallel circuit, the voltage of all batteries within that circuit must always be the same as the rest.

· Therefore, any discharge (or conversely, recharge) will be governed by voltage.

· If there is a lower capacity battery in the circuit, only enough capacity will be withdrawn to discharge that battery to the same voltage as the rest of the circuit.

· If, for example, there is a battery that has 50% of the capacity of the rest, this battery will not produce as much capacity but it will not be discharged to any lower voltage than the rest of the circuit, thus protecting it from over discharge and subsequent reversal.

[tinlizzie23]

Good info. Thanks for posting it.

[skymutt]

I'm thinking that you would only connect in series if you have two 6V batteries and want 12V. Two 12V battries in series would produce 24V wouldn't it?

Nick

[Rustic Eagle]

Yes.

Bob

[Mustang65]

It pretty much generic, whether you are talking 6 volt series/parallel, 12 volt series/parallel or 24 volt...... Granted RV's are 12 volt, so 6volt batteries could use both series/parallel depending on the number of batteries, as for 12 volt batteries, series would not apply only parallel, but the concept holds true for any combination of series/parallel setups.

Don

[David472]

Keep in mind, each 12 volt battery is made up of 6 individual cells connected in series. Two volts each. The same principles apply when one cell is weaker.

[OnTheGo]

Originally Posted by tinlizzie23

Good info. Thanks for posting it.

X2 - Mustang65, thanks for doing the research on this one. There are legitimate technical reasons things work well (or don't) and we don't always know what they are. Your post did an excellent job with the new / old battery issue - that we have gone over a time or two.

[Bassdogs]

As a general rule, doesn't it make sense to swap out all the batteries at the same time? That way the batteries should begin to fail at about the same time baring a quality problem with one of the batteries. Long term the $$ is the same, and it gives you confidence knowing that the battery pack has been upgraded rather than just one of two.

I have used the 6 volt golf cart batteries in both my TT and houseboat. I plan on upgrading the 12v that came with my whitehawk with 2 6v's in the Spring. Will be plugged in on our trip south in about a month so the single 12v should handle the road trip down and back.

[Chakara]

This is interesting. Makes me feel bad for asking Costco to replace both of the 12V in my Ram when one battery lost a cell at 18months old. (BTW, Costco rocks.....18 months into those batteries they replaced both with no charge.)

I use 2x6V on the trailer so will be stuck when one fails. Oh well, worth it for the capacity vs space on my trailer!

-Chak

[robkelly]

Thanks for sharing! Especially good for a newbie...