gav- Posted May 3, 2008 Share Posted May 3, 2008 I found that the battery was dead today on my p38a, i thought it may have been due to the RF amp and all the wireless stuff thats around my area. so i unplugged it and charged the battery back up, to the point i was able to run the engine again. Checked the battery every hr and it was loosing charge. put an ammeter across the terminals and there was 19A across it. i investigated further, and got to the point where I disconnected the battery cables and still found i had 19A across it! Does anyone know the mechanism for this to happen? Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted May 3, 2008 Share Posted May 3, 2008 You want to be measuring the voltage across it - not ampage. The ampage is a meaure of current flow through the ammeter - this can give an indication of the condition of the battery, but it won't change if you disconnect the wires. After a battery has charged, it will take a few hours to settle down. Let it settle for a little while before checking the voltage across it. It should remain constant (to the nearest 0.1v) It is possible that the vehicle is draining the battery - do the above test with the battery disconnected. If you find it looses charge with it connected, there is an electrical fault on the vehicle. Quote Link to comment Share on other sites More sharing options...
gav- Posted May 3, 2008 Author Share Posted May 3, 2008 i have been measuring voltage. it would drop to 4v when left for a day. i did wonder if the ampage i was reading was due to the ammeter, but 19a seem alot to me, thats why i am asking the question. is it possible for a battery when out of the vehicle to have a load on it?? (my knowledge seems unlikely but checking) Quote Link to comment Share on other sites More sharing options...
Kim Horsevad Posted May 3, 2008 Share Posted May 3, 2008 I found that the battery was dead today on my p38a, i thought it may have been due to the RF amp and all the wireless stuff thats around my area. so i unplugged it and charged the battery back up, to the point i was able to run the engine again. Checked the battery every hr and it was loosing charge. put an ammeter across the terminals and there was 19A across it. i investigated further, and got to the point where I disconnected the battery cables and still found i had 19A across it! Does anyone know the mechanism for this to happen? Please dont use an am-meter that way... An am-meter works by measuring the current flowing through it. If you were to connect an am-meter directy to your battery you would effectively short-circut the battery. Such instances are known to produces large sparks, fires and obscene amounts of rather unhealty fumes.... Use a Voltmeter to measure the voltage. For at fully recharged battery it should be around 12,5 volt. Connect the battery to the vehicle and run the starter for 5 or 6 seconds (remove the power supply for the fuel solenoid). Then use the voltmeter again. If the battery voltage has dropped significantly (more than 0,5 volt) the battery is showing signs of age and should be replaced. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted May 3, 2008 Share Posted May 3, 2008 If you need to measure standing current from the battery, remove one of the terminals and connect your ammeter between that terminal and the battery post you just disconnected. Depending on the type of meter, observe polarity. Don't try to start the car with an ammeter connected this way! Quote Link to comment Share on other sites More sharing options...
SteveRK Posted May 3, 2008 Share Posted May 3, 2008 If the disconnected battery is going flat in 24hrs it must have an internal short circuit on one or more cells. Typically this is caused when a plate crumbles and sediment builds up at the bottom of the battery shorting one or more plates together. Shocks and vibration hasten the demise of batteries in this way Quote Link to comment Share on other sites More sharing options...
gav- Posted May 3, 2008 Author Share Posted May 3, 2008 thanks for the replies, i havent put any additional load on the battery when measuring it. however i have realised my mistake, which i am hoping is just due to a rough day! of course a volt meter measures across the circuit where as a ammeter is in it. silly boy! Quote Link to comment Share on other sites More sharing options...
petrolhead63 Posted May 3, 2008 Share Posted May 3, 2008 I think the breakdown of the plates is called calciification, or simliar! If you have a good old battery you can take the top plugs out you can on old batteries sometimes see the crust between the plates. I did myself have a battery go totally open circuit......I have never heard of one creating its own current although I think the plates shorting out do that basically. You can also watch the voltage drop whilst cranking and it shoud hold up for some time with a good battery. (I can check the rough expected voltage to hold up if you wish noted outside in the garage!) Quote Link to comment Share on other sites More sharing options...
JimAttrill Posted May 5, 2008 Share Posted May 5, 2008 I have heard many complaints about P38s and batteries. The car needs a high voltage to work and this nadgers the battery a bit quicker than normal. So it could just be another duff battery. Many problems with P38s are caused by lowish battery voltage, less than about 13.5 causes problems. Quote Link to comment Share on other sites More sharing options...
Boothy Posted May 5, 2008 Share Posted May 5, 2008 Batteries have changed significantly in the metals and amounts of metals used in recent years (due to the lack of lead and dislike of use by anti-lead brigade) which has a detrimental effect on normal batteries, also when a battery fails nowadays it's an instant event and not a series of near misses and slow starts on cold mornings, more like a light bulb going kaput. I use massive submarine type maintainable batteries for high output current usage (big loads at remote locations) at work and its becoming more difficuly to source and use a good quality battery that will be reliable and maintainable. I think gel batteries are the way forward but pricey. And please be carefull when testing batteries on Amps for your sake and the meters sake, make sure they're well vented, we had a lad blow his beard of one day when testing on amps by mistake, they exploded because of gas buildup. Smelt bloody horrible. Quote Link to comment Share on other sites More sharing options...
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