Quick battery charge testing?

[Morich]

We get lots of posts and lots of really techie ideas on how we know if the various battery charging systems are working and it often seems to get more difficult and complex than needed, if we only want to know if they are working.

I thought I might lay out what I see as a much easier, simpler, and more definite method to test these things.

We have two systems of 12VDC battery, one for the normal truck/ car functions and one for the RV portion and while they are separate systems, they are also connected at times for different reasons.

The normal car battery charging seems a good place to start as we are more likely to be used to that area.

First step on understanding any battery is that the cables have to be clean and make good contact, both on the negative lead which goes to the frame for ground and the positive lead which supplies power to the breakers or fuses.
Those cables have to make good contact at BOTH ends. So test the connections!
Simple if you have a meter to test voltage and resistance or ohms (the funny little upside down horseshoe?)

For resistance testing, put one meter lead to a good metal part that is ground and the other lead on a metal part of the clamp at the battery negative post, not the post itself yet, and you should read near zero. If not, the cable at the other end is likely to need cleaning. Then when you have a good clean connection to ground, move that meter lead to the negative battery post and get the same reading to prove you have a good clean connection at the battery post.

Once you know the ground cable is good, move over to the other set of batteries and do the same test to make sure both the start and coach batteries are grounded well.

With the RV unplugged and the engine not running for an hour or more, we can then test the battery voltage. Switch the meter to DC voltage setting and somewhere just above 12 like 20 volts scale. Leave one meter lead connected to a good ground and test one set of batteries at the positive post itself. Note that you should have about 12.6-12.8 on a fully charged and stable battery. Any higher and you are seeing the "surface charge" left from the charging system as batteries have a "natural level" no higher. If you see higher, you can still test, just know that it is a false reading and it will go down given more time without charge.

Once having the post reading, move to the clamp on the positive post and you will see the same if the connection if good and clean. Test the other battery string the same way.

Now you can move to testing the charging items.

Consider writing down each battery voltage to compare as you test. If the start and coach batteries read a bit different, that makes it easier!

To test the charging when driving down the road, put the meter on volts and connect it to the positive post or cable of the coach battery and start the RV engine. When things are working right with the mode solenoid, the engine alternator puts a charge voltage out to the start battery and the solenoid connects that charge voltage to the coach batteries and you should see that change on the meter! If you see 12.6 and then 13 or more when the engine runs, your coach batteries are charging, so move the meter over to check the start battery is doing the same. If you rev the engine up, you might expect to see the reading go as high as 14 volts. If both readings jump up when you start the engine, that system is good.

The second system is the charging for the RV batteries for use when we are parked and it gets confusing because it can work kind of opposite of the first system! Let the batteries rest for a few hours for best readings and easier to spot the changes, but you can often spot it when done right away after the first tests. Just be aware that the battery voltages will be changing as you are using some while adding some to the voltage readings.

When ready and you feel the batteries are stable again, ( a couple hours is nice) set the meter on voltage, connect one lead to ground and one lead to any part of the coach battery system and note the reading while not plugged into power. Then plug in and expect to see the reading go higher. This is where letting the batteries get settled will help as they will go back below 12.8 and it is easier to spot when the charge voltage shows up at 13 or more!

If you see this voltage jump, your converter is working!

If you have the battery disconnect switch turned on, the 12VDC inside the RV should all be working.

But then we have this little battery boost switch thing that we need to check since we are here. It is to use if the start battery doesn't crank the engine well and may be getting a bit weak so we want a "jump start" by simply pushing the boost button.

To test the wiring, switch, and mode relay, we don't have to go dig into the tough stuff, just watch the battery voltages!

You've tested the coach battery charging and found it working, so move the meter over to the start battery and you should see the voltage lower than the coach battery, since that one is being charged by the converter. Maybe coach at 13 and start battery at 12.6 or less? Having a helper is handy, so have them push the battery boost switch a few times while you watch the meter. reading on the start battery.

If the start battery voltage jumps when the boost switch is pushed the mode solenoid is doing what it should and connecting the two strings together temporarily.

Looking back, you have tested the battery and cable connections, battery cutoff switch, mode relay, and boost switch without having to dig through the whole coach to find them!

If you have done this correctly and in the right order, there is no more to test. If you do not get the changes at these points, check your testing and if still not getting the right answers, THEN consider you may have an actual mechanical problem and begin to sort that question.

[DavidM]

Morich:

Good tutorial on checking your battery systems. But I have one quibble:

Vehicles at least for the last several decades, do not use the vehicle chassis for a ground. All wires are run in pairs, positive and negative to the device. They never use the chassis ground as the negative return. Most do connect the negative to chassis ground at one spot but they don't rely on the chassis ground to power any circuits.

RV coaches are similar. They may connect the negative lead to the RV's frame like vehicles, but even more so than vehicles which typically have steel bodies, RVs are often fiberglass. So they do not rely on the frame as the return negative circuit.

So look for a negative buss near your coach's batteries as well at the chassis battery. In many cases it won't be a buss, just several wires screwed down to the negative terminal to the battery.

Anyone who has dealt with old vehicles and has cleaned up poor negative connections to the steel body (typically at tail lights) knows why the chassis ground is no longer used. Thank goodness!

David