"Low" engine battery voltage, but everything generally runs fine?

dbsoundman · 01-03-2022, 08:20 PM

Trying to diagnose some oddities in my electrical system (engine batteries)... Batteries are new, purchased from a battery wholesale place in June 2021. I can start the bus just fine, every time, but the battery voltage reported from the diagnostic port and on the dash is always low; never over 11.9 volts with key set to ON, engine off, and always 13.9 volts with the alternator running. Measured the battery voltage in the accessory panel with the engine off, key off, I definitely measure 11.9 volts or less (depending on how long the bus has sat), so the dash and diagnostic port measurements are accurate.

As far as I have seen thus far, the batteries themselves measure just fine (12+ volts).

I tried disconnecting, cleaning, and reconnecting the main battery connection to the frame, as well as several other connections to the frame closer to the engine itself, but haven't seen any improvement. I did find the ignition solenoid and the voltage is the same on either side of it so that's not it either.

Is this "low" voltage reading actually OK? I feel like I'm chasing something that may be a non-issue. Everything I've read says that a battery voltage lower than 12.3 or so is no good. What else should I check?

I should note that the bus runs and drives great, but I've noticed a few oddities: 1. the turn signals/hazards flash a little fast (replaced the flashers, same problem); 2. the dash gauges don't work at all while the engine is running.

Spaznaut · 01-03-2022, 09:10 PM

Get those batteries charged up and tested. You can get bad batteries. It shouldn't affect the dash issue though.

dbsoundman · 01-03-2022, 09:18 PM

Quote:

Originally Posted by Spaznaut

Get those batteries charged up and tested. You can get bad batteries. It shouldn't affect the dash issue though.

I have a 6 amp charger so it's charging them up...slowly but surely.

I didn't think the dash issue was related, but I wondered if maybe the dash went crazy if either it had a bad ground, or if the battery voltage was consistently too low for it. Seems like it would have to tolerate low voltage though, so I'm guessing I just have an unrelated bad dash problem.

TheHubbardBus · 01-03-2022, 09:57 PM

I'm confused by where you've taken the readings from. Have you measured the batteries with a voltage/multi-meter at the battery terminals? Is it still 11.9 if you do?

11.9 is very low. Pretty much dead.

A 12V lead acid battery isn't really a 12V battery. Fully charged @ 77-degrees, a healthy battery should read around 12.7-12.8V. At 12.3V you're down to ~50% capacity. And again, 11.9 is basically fully discharged. Low temps will lower those voltages, but not that low.

The 13.9 is what the alternator is producing, and that's in the ballpark of what you'd want to see for its output, so that shouldn't be the issue.

Something doesn't add up. I can't see the bus just starting up like it ain't no thing @ 11.9V.

bubb, the real one · 01-03-2022, 10:02 PM

What hub said,
Disconnect battery from bus, Charge batt overnight, disconnect from charger for a few hours and then test from the terminal

cadillackid · 01-04-2022, 07:04 AM

if your engine has an intake heater it will turn on any time you turn the key to 'ON' and it stays on for a few minutes if i remember right.. that lowers your displayed voltage down as the system is pulling a solid Load..

dbsoundman · 01-04-2022, 07:06 AM

Quote:

Originally Posted by cadillackid

if your engine has an intake heater it will turn on any time you turn the key to 'ON' and it stays on for a few minutes if i remember right.. that lowers your displayed voltage down as the system is pulling a solid Load..

I think it does have one, but the vintage usually goes up a couple tenths of a volt once the WAIT TO START light goes out, which makes sense.

Oldyeller · 01-04-2022, 11:46 AM

I agree with TheHubbardBus, check the alternator output at the battery with a VOM. Who knows what might be happening with the wiring to the connector you've tapped into. I also agree that if you ACTUALLY had battery voltage of 11.9 the bus wouldn't start.

rffffffff · 01-08-2022, 08:47 AM

It’s really hard to make any conclusion about battery state or what’s high or low without understanding the load on the battery and the loss across the wiring.

If you’re at 11.9 but there’s a 30a load on the battery (for example) your batteries may be fine and the load is pulling the voltage down.

If you want to diagnose this I would first acquire a dc ammeter clamp that you can put around a battery wire and determine the load being pulled through it. If it’s a big load it’s likely an accessory drawing a lot of current and may or may not be fine. If it’s minimal you have another issue.

Secondly, if your voltage is 11.9 at the battery and the accessory panel and everywhere else the issues is systemic and probably load related. If you’re at 12.6 at the battery and 11.9 at the panel you have a bad connection somewhere in between. I’d love to know the voltage difference between the accessory panel and the alternator while running too. same deal, if it’s all the same you’re probably fine.

Assuming the voltage is universally low I think the clamp meter will help you figure out what’s pulling so hard and you can isolate that component by clamping around wires in the accessory panel to start. Hopefully that’s a little helpful.

dbsoundman · 01-09-2022, 06:43 PM

Quote:

Originally Posted by rffffffff

It’s really hard to make any conclusion about battery state or what’s high or low without understanding the load on the battery and the loss across the wiring.

If you’re at 11.9 but there’s a 30a load on the battery (for example) your batteries may be fine and the load is pulling the voltage down.

If you want to diagnose this I would first acquire a dc ammeter clamp that you can put around a battery wire and determine the load being pulled through it. If it’s a big load it’s likely an accessory drawing a lot of current and may or may not be fine. If it’s minimal you have another issue.

Secondly, if your voltage is 11.9 at the battery and the accessory panel and everywhere else the issues is systemic and probably load related. If you’re at 12.6 at the battery and 11.9 at the panel you have a bad connection somewhere in between. I’d love to know the voltage difference between the accessory panel and the alternator while running too. same deal, if it’s all the same you’re probably fine.

Assuming the voltage is universally low I think the clamp meter will help you figure out what’s pulling so hard and you can isolate that component by clamping around wires in the accessory panel to start. Hopefully that’s a little helpful.

All very valid points! The way the bus is currently parked in my driveway I can't get the battery box all the way open so I'll have to investigate in a few days. I've always wanted an excuse to get a clamp ammeter so I guess now's my chance!

The 6 amp Schumacher charger took about a day to charge the batteries up to 100%, it was saying they were like 65-70% when I first turned it on. I've left it on in battery maintainer mode ever since. I connected it inside the accessory panel beside the driver's seat so it's easy to hook up to the maintainer while it's inside the bus.

I think my dash issues are the actual dash itself, I was hoping maybe I just had a bad ground somewhere affecting the dash but I'm thinking the main "brain" of the dash is just toast. I almost wish my bus was just a couple years older, I think this is the first generation "digitally controlled" dash for Thomas. All of the sensor wires coming in to the dash are analog, but the actual gauges are driven by a module with a multifunction LCD display for the odometer, etc. I think that piece is what died and has left me with no working gauges and some warning lights that never shut off.

peteg59 · 01-09-2022, 07:07 PM

Can't help you with your voltage issue but the turn signals flashing a little fast could be a bad bulb, or someone has installed LED bulbs in place of the standard incandescent?

Good luck on troubleshooting, and let us know what you find on the other issues...

Nick969 · 01-10-2022, 11:56 AM

My batteries drain down over a few days sitting not being started. I cant find the cause. can i put a switch on the battery to disconnect it from bus when sitting so as to not run down the batteries without creating any issues with computer etc?

rffffffff · 01-10-2022, 12:14 PM

Quote:

Originally Posted by Nick969

My batteries drain down over a few days sitting not being started. I cant find the cause. can i put a switch on the battery to disconnect it from bus when sitting so as to not run down the batteries without creating any issues with computer etc?

Yes, or get a solar charger that you can leave attached if the bus is stationary.

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