*Bad Hydrometer Or Battery?*

And Why Not To Mix & Match Batteries In A Bank

By Jaden

16 April 2004


Iíve been having some problems with my batteries readings. After a full charge and a float voltage of 12.8ish the hydrometer would read around 1200, which is in the discharge state. I got the battery voltage up over 13 volts and it read the same. I thought something was seriously wrong. I thought I might have a bad hydrometer but then 2nd guessed myself why I tested a separate battery and got an OK reading. The battery bank was working fine, holding near perfect voltage and running the inverters. So I put a question up on the board describing the problem.

Hereís what the electrical engineer had to say-

First, It sounds like you have a cheap hydrometer.. they are NOT created equal.

When you are charging a battery, there is point in time where it's "Full", it can not handle any more voltage.. this is it's normal charge capacity. After the battery cools down a bit from charging, it'll handle a little bit more voltage, so you trickle a little extra current into it and that's called "Peak voltage" (Called "Peaking the battery")

When in use, Batteries discharge over time at a predictable rate. Each battery will have it's own unique discharge curve. What you want is batteries with similar discharge characteristics which is why you don't run an Everstart, an Everready, a Diehard, etc on the same bank.. you want all of your batteries to be the same brand name, same product number, and preferably the same lot number.

A battery will hold it's peak voltage under load for only a short time.. the "Peak" drops off rather quickly and then you have long period of time where the curve flattens out. The battery may drop it's peak in a few minutes, but holds it's run voltage for several hours., this is normal, all batteries do it.

You want your batteries to give you that Loooong run time, and then "just die".. cheap batteries will often just keep getting weaker and weaker until they dim away.. you want batteries that will hold their run voltage for several hours, then suddenly completely die within a few minutes.. then you know you've gotten all that the battery has to give.


I feel a lot better about my bank than I did before. Still keeping a close eye on it. Thatís what I learned today.


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