Poor battery life

[Gareth Covell]

I got the idea for the bar graph's from evconvert.
The description of how to make them is very good, so some weeks ago I made one.
The bar graph worked but the voltage span was only about 1.5v, so if I set the top led to come on at 13v the bottom led would come on at 11.5v. The span I was looking for was around 3.5v.
I got in touch with James May who wrote the evconvert article and he was very helpful.
He told me which two resistors I needed to replace in order to increase the voltage span.
The changes to the original design are;
R1 56k (this is for 12v batteries).
R3 270 ohms.
R4 500 ohm variable ( this is instead of the 200 ohm variable resistor ).
R5 two 270 ohm resistors in series ( this makes 540 ohms ).
By changing R3 and R5 it is possible to have a larger or smaller span, however the total resistance of R3 R4 and R5 must = about 1300 ohms, don't know why but James May told me that this is the case.
I bought the parts from RS components the cost per unit is aprox. £7 plus p&p.

[casemod]

Even for a PBA battery you should use some kind of BMS/PCM to monitor individual voltages and if possible balance the cells. Its impossible to correctly control the charging voltage of the batteries when they are in series. You may have 13V in one and 15.8 on other. Same while discharging, one may have with 11.8 and other with 8.2V (one element inside reversed).

If only one battery in the string is damaged you are no longer being able to monitor individual voltages if you only look at your pack voltage and the result is over/under discharged cells in the pack. Now instead of replacing one faulty cell they all get slowly damaged.