battery discharging..alternator?

[kathyshack]

Hi,
Just a quick one. The battery on my brothers van keeps discharging itself. We had thought this was because it had been stood a while etc, so we put a new battery in. This doesn't seem to have solved the problem however. Is there a way of checking whether the alternator is working or not? (This is what charges the battery when driving isn't it?). Van works when battery has been charged up, but rapidly looses it....
Thanks in advance
Kathy

[tonytech]

Connect a Voltmeter across the battery ie +ve to +ve and -ve to -ve.

Read meter. Voltage should be about 12 - 12.5V for a charged battery.

Start Van. Rev to 1500 - 2000rpm. Read meter Voltage should be 13.5 -14 V



T

[kathyshack]

brilliant, thanks!
so if voltage stays at around 12v then alternator aint wokring as it should?
tha van doesn't have a rev counter, so how do i know if i'm reving it enough?
is there anything else which could cause the battery to discharge quickly like this? There is a split charge relay installed which we don't know if its working or not as never used it. (leisure battery dead). Had the van for 12 months now though and we never had this problem until recently, which makes me think it can't be this thats casuing it..

[cumbriankeith]

Voltage should be about 12 - 12.5V for a charged battery

a charged battery reads 12.7 or so - 12.5 is quite well down. We also need to remember that you can only take this reading when the battery has been used a little after charging (ie the engine has been running) to get a true reading of voltage and therefore state of charge. Temperature also has an effect...


This website has more:
Because batteries are always referred to as 12-volt (or 6V or 24V), it is often assumed that the normal voltage is 12V. In fact a '12V' lead-acid battery only producing 12.0V is either almost flat or is delivering a large current (under a heavy load). In fact, a healthy '12V' battery, when not being charged, should always show 12.2 - 12.8V.

Very conveniently, the relationship between a battery's state of charge and its voltage is linear (10% per 0.10V) between about 90% (12.70V) and 20% (12.0V). However, when a battery has been on charge, even if it's not fully charged, the voltage will be up around 13.8V. This will slowly drop to around 13.6V over the next few hours, but even if left overnight it shouldn't drop below 13.0V unless the battery was only partly charged or is on the way out. But, as soon as a reasonable sized load is switched on, the voltage drops quite quickly until it equates to the battery's actual state of charge. Thereafter, the rate of voltage change (under constant load) becomes linear. Thus, in practise, the voltage of a freshly charged battery is a poor guide to its state of charge until some of the charge has been used. With a conventional internal alternator regulator, the best guide to when charging is as complete as it's going to be, is when the voltage at the battery remains constant for ten minutes or so at the regulator's maximum of around 14.4V. If it never reaches 14.2V, first check both sides of the charging circuit for poor connections. If that doesn't solve the problem, have the alternator checked over and the regulator changed if necessary. 14.4V is the point beyond which gassing starts to occur under prolonged charging (continuous motoring), but the closer to it that the regulator gets, the faster and more complete the charge will be.

[tonytech]

First thing disconnect the split charge relay at the main battery.
If you are trying to charge a buggered battery you will be getting less charge into your main battery.

Just get your revs above the idle. Most times idle revs wont charge a battery propperly.

Yes a fully charged GOOD battery wil give a open circuit voltage reading above 12.5V but my (almost new) Main battery gives a average voltage reading of 12.45V (engine off, clock and radio standby drawing current)


T