Batteries : How to test?

[neilbardsley]

Before more knowledgable DDKs helped on Saturday morning I was faced with trying to drive back to the UK on my battery (without recharge). Now I have a good quality battery red top 6v and but I decided to do some research to see if I would of made it. This web site is useful

http://www.dcbattery.com/faq.html#9

It's appears that the two most important ratings are

CCA : COLD CRANKING AMPERAGE :
A 550 CCA battery can supply 550 amperes for 30 seconds at 0°F

RC : RESERVE CAPACITY :
Reserve capacity is the number of minutes a battery can maintain a useful voltage under a 25 amps discharge

Also useful is

Ah : Amp-Hour
If you 100Ah then your battery will discharge 10amps for 10 hours. However, this isn't as useful as the RC because the manufacture can choose the amp they test at to suit their battery and not your car.

CYCLE LIFE

The number of times your battery can go from full charge to full discharge and a return to full charge again

My Red Top Optima has a RC of 100 and Ah of 50 which means if I had had the full capacity then lets say I was using 5 amps I could of got about 10 hour worth but if I had the lights on, using 15amps, maybe I would of got 4-6? Perhaps I would of got home but I wonder at what point it wouldn't have enough charge to start the car. This is assuming my battery still performs as well as when it left the factory!

Is there anyway to test the health of my battery without doing a full discharge test?

According to the information on the page over charging is one of the best ways to damage your battery? I have my battery on continous drip feed from a solar panel. How do I know if I'm over charging it?

[DIRKBROWNFINGER]

Is it a charger or conditioner you are using?

[neilbardsley]

Is it a charger or conditioner you are using?

It's a solar charger

[chris68]

Hi Neil,

Normally, provided the battery is in good shape, open circuit voltage (i.e. nothing connected) is a good indication of the charge state. Something approaching 12.8V indicates a fully charged battery and 11.0 means it's dead flat.

However, you can have the situation where the internal resistance of the battery has gone high for some reason. This means that as soon as the battery delivers current, the voltage across the terminals falls due to internal volt drop. To measure internal resistance you can record the open circuit voltage then connect a known load and record the on-load terminal voltage. Work out the difference between the two voltage measurements and divide by the load current to calculate internal resistance. Somewhere around 0.07ohms is probably normal for the small batteries used on our cars but check the datasheet for your battery. Bigger batteries will have lower internal resistance.

Continuous drip feed should be fine provided the charging voltage doesn't exceed about 14.0V.

Cheers,
Chris