MontanaChinook Posted February 20, 2015 Share Posted February 20, 2015 If I wanted, for the sake of curiosity, to check what my alternator is actually putting into my battery, how would I perform that test? I'm only just starting to understand enough about electrical systems to feel that I want to know these things... I have a brand new coach battery, and I'm just curious to know how efficiently my truck is going to charge it, so it doesn't die an early death. So this is not so much an alternator output test, but a test to see how much of that charge is actually getting to my battery, through whatever wiring nightmare might connect them. Quote Link to comment Share on other sites More sharing options...
Maineah Posted February 20, 2015 Share Posted February 20, 2015 Common old voltmeter will do it. First check the truck battery's voltage then compare it to the coach battery's voltage while it's running they should be fairly close the coach probably will be slightly lower (10th's of a volt). Quote Link to comment Share on other sites More sharing options...
zero Posted February 20, 2015 Share Posted February 20, 2015 If I wanted, for the sake of curiosity, to check what my alternator is actually putting into my battery, how would I perform that test? The job of the alternator and regulator is to keep your battery (or batteries) at 13.5 - 14.2 volts all the time, as long as the engine is running. That is supposed to happen regardless if all the lights and appliances are on, or all are off. If a voltmeter reads within that range with nothing on, and does the same with most everything on, the alternator and regulator are working fine. Quote Link to comment Share on other sites More sharing options...
Maineah Posted February 21, 2015 Share Posted February 21, 2015 An alternator is voltage dependent so if the voltage is there so is the current needed to keep there. Quote Link to comment Share on other sites More sharing options...
MontanaChinook Posted February 21, 2015 Author Share Posted February 21, 2015 So simply put one lead on the positive post of the battery, and the other on the negative? I guess my question is as much about the actual process than just which component to test. I own a multimeter but have yet to use it. I can find instructions on how to use it all over the internet. Just wanted to know where to actually place the leads to test. Sounds like the battery terminals are where I test. I'm more concerned about voltage drop in the wires between the alternator and battery than I am the actual alternator output, but I guess first I'll see if the battery is being held at the right voltage. If it is, then it doesn't matter. If it's not, then it's time to find out if it's the alternator that's the problem, or the wiring, correct? Thanks. Quote Link to comment Share on other sites More sharing options...
WME Posted February 21, 2015 Share Posted February 21, 2015 Measure V at the truck battery, measure V at the coach battery. Subtract coach V from truck V and that is your V drop. Now measure V between the input terminal of the isolator and a clean ground, V at the truck terminal and V at the coach terminal. This will tell if the isolator is OK. A diode isolator will have a .7v drop across it, a properly working solenoid type will have a much smaller drop. The V drop between isolator and the coach battery is the charging wire so just look at its entire length. When I bought my Escaper there was a 1.5v drop between the isolator and house battery. I found 3 different size wires and 4 splices on the charge wire. I replaced with a single run of #6 and had a .15v drop and my house battery was much happier. Quote Link to comment Share on other sites More sharing options...
MontanaChinook Posted February 21, 2015 Author Share Posted February 21, 2015 Yeah there's some serious splicing (and not too pretty looking) on those wires going into the isolator. That's part of the reason for my question. Thanks. Quote Link to comment Share on other sites More sharing options...
zero Posted February 21, 2015 Share Posted February 21, 2015 I'm more concerned about voltage drop in the wires between the alternator and battery than I am the actual alternator output, but I guess first I'll see if the battery is being held at the right voltage. If it is, then it doesn't matter. If it's not, then it's time to find out if it's the alternator that's the problem, or the wiring, correct? The charging system is designed so the alternator senses battery voltage, NOT voltage at the alternator output. So that voltage drop in factory wired systems is taken into account. But in the case of an RV where the alternator charge wire has been cut and hooked to an isolator - the system often does not work exactly as designed. Regardless - a 12 volt battery is around 12.7 to 12.9 volts when fully charged. Any voltage that is higher will charge it. Most regulators for 12 volt systems are designed to charge at 13.8 to 14.2 volts. Take your voltage reading at the battery posts. Try it with the headlights off, engine idling. Then with the headlights ON and engine idling. If the voltage at the battery stays over 13 volts you are fine. Quote Link to comment Share on other sites More sharing options...
Derek up North Posted February 21, 2015 Share Posted February 21, 2015 And if you really want to test it, switch on high beams, maximum heater fan, wipers, etc. Quote Link to comment Share on other sites More sharing options...
MontanaChinook Posted February 21, 2015 Author Share Posted February 21, 2015 Alrighty. Thanks. It'll be a bit since the head is at the machine shop right now, but it'll all be put back together mid-March, so not too long from now. Quote Link to comment Share on other sites More sharing options...
Maineah Posted February 22, 2015 Share Posted February 22, 2015 If the voltage seems low before you replace any thing make sure the alternator drive belt is tight. Quote Link to comment Share on other sites More sharing options...
MontanaChinook Posted February 22, 2015 Author Share Posted February 22, 2015 It is. I re-tightened it last summer, about a year after putting a new belt on. Quote Link to comment Share on other sites More sharing options...
Maineah Posted February 22, 2015 Share Posted February 22, 2015 Good, those things need to be good and tight on the 22R/E's It is not a good thing to have splices and such on your battery wires the stock system is a #8 wire so if it's funky I would just start from scratch with a #6 as a replacement the price difference is minimal however there are millions of motor homes and campers out there working just fine with a #8 wire. Yes check the voltage at the battery posts + and - by using the posts and not the cables might show you a voltage drop that actually is good if it is low the first step would be to clean the cables and the battery posts. Quote Link to comment Share on other sites More sharing options...
MontanaChinook Posted March 15, 2015 Author Share Posted March 15, 2015 I haven't done all the tests you guys suggested, but while I was letting it run today I pulled out the multimeter and checked the voltage at both batteries. 13.75 at the truck battery, 13.9-ish at the coach battery. I'll try those other tests in the next couple days. Was just doing a quick check while it was running but I was more concerned with making sure my engine wasn't eating itself after I put it back together. On the plus side we just had a week of 50s and 60s so all the stuff I was thinking I'd have to do in 20 degree weather has been really so nice to work on, out in spring weather and sunshine. Quote Link to comment Share on other sites More sharing options...
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