"On Charge" voltage for 6V battery

[UroGator]

What should the on charge voltage be for a 6V battery? I bought a used cart with 5 Trojan T-605s (Dec 2000 stamp) and 1 shorted interstate battery. I couldn't find any used batteries, so I bought a new Trojan T-105 to replace the bad Interstate. I can get my final resting voltage up to 37.9 V. however, what I noticed today was that while on charge, the new Trojan was reading 8.4 V while the other five old Trojans were reading 6.7V.

At rest here are my following readings

6.40- new battery
6.35
6.35
6.25
6.25
6.35



I realize that my best option is to bite the bullet and replace the other five Trojans but I wanted to see how much life I could get out of the other batteries. My question is how much harm in my doing to my new battery by what appears to be "overcharging". How many fewer cycles can I expect if I do this for saying another month before I can get five new Trojan T105's?

[JohnnieB]

Unless you know for sure the other five batteries are over 13 years old, I suspect the "L0" date code means Dec-2010.

Adding a new battery to a 3 year old pack is bad, adding a new T-105 (225AH) battery to an old T-610 (210AH) pack is even worse.

The On-Charge voltage is determined the charger as well as the SoC of the battery being charged. They are connected in series, so the same number of amps flows through all six batteries and if they are close in age and AH capacity, the On-Charge voltage will be about the same on each battery in the series string. About 1/6 of the voltage the charger is putting out.

The charge profile for a Trojan battery has 2.79VPC (Volts per Cell) being the max recommended On-Charge voltage, which is about 8.37V for a 6V battery, so 8.4V is on the ragged edge of being too high.

An At-Rest voltage 6.4V is about normal for a new Trojan battery, so you may not have harmed it too much yet, but you definitely should replace the five old T-605 batteries with new T-105 batteries with all deliberate haste.

[UroGator]

Thanks for that explanation JohnnieB. You are the man.

The actual charging voltage of that T105 was 8.37, I just rounded up to 8.4. Looks like I just have to bite the bullet now and buy the other five T105's

[JohnnieB]

You might get by for a month without harming the new T-105 much, but a new battery added to an old pack ought to be considered a sacrificial battery, so the sooner the better.

The problem is that a new battery ages quickly in an old pack and a single old battery will make an otherwise new pack age more quickly. Basically, any harm done to the new T-105 will be forward to the five new batteries that replace the existing T-605 batteries.

Batteries are akin to apples. Put a good apple in a bushel of bad apples and it quickly goes bad and if you put single bad apple in a bushel basket of good apples and you soon have a bushel of bad apples.

[UroGator]

Originally Posted by JohnnieB

You might get by for a month without harming the new T-105 much, but a new battery added to an old pack ought to be considered a sacrificial battery, so the sooner the better.

The problem is that a new battery ages quickly in an old pack and a single old battery will make an otherwise new pack age more quickly. Basically, any harm done to the new T-105 will be forward to the five new batteries that replace the existing T-605 batteries.

Batteries are akin to apples. Put a good apple in a bushel of bad apples and it quickly goes bad and if you put single bad apple in a bushel basket of good apples and you soon have a bushel of bad apples.

Thanks. I like the apple analogy.