Potential Voltage Issue? Voltage gauge jumping around.

[radrob]

Hi everyone, I am new here and this is going to be my first post. I am brand new to the world of Humvee's, but not new to the automotive world.

Recently we acquired a '92 Humvee and it was in great condition mechanically and cosmetically. When we first acquired it, the truck was a little sluggish to crank and fire, but it did so just fine. I was planning on adding a bunch of electronics to it for some testing purposes, so I decided to just go ahead an swap out the batteries. I went ahead and installed 2 new Optima yellow tops, and rewired them the same way the old ones came out. Everything was completely fine, except for the battery voltage gauge would jump around on initial start up. It jumps around for about 5 minutes before settling down and stays solidly in the green right where it needs to be. Now if its night time and the headlights are on, they flicker with the gauge jumping around, but once everything settles down, they stay nice and steady. Being that this was just a test vehicle for us to develop some products for, I didn't think much of it as even if it sat for a week or 2, it would crank right up no issues and not even slightly sluggish.

Fast forward to last week, some coworkers took it to lunch, and within the hour of it sitting in the parking lot, the batteries had drained enough that it was cranking slowly and wouldn't start. They called me and I made sure they weren't messing around with the lights or something and left them on, and they assured me that they didn't.

So this morning I came in ready to start diagnosing some issues. I thought after doing some research on this site it could be a grounding issue, voltage regulator issue, or an alternator issue. First thing I did was fired it up and let it sit and run for about a half hour just to get it all warmed up and make sure the alternator had time to charge the batteries back up. It cranked slowly, but it did fire on its own. After the 30 minute idle, things are back to normal and it cranks up with full power.

So here is where my questions come in. When taking a volt meter to the batteries while the vehicle is not running, one of the batteries is reading 12.9 volts, and the other battery is reading 11.3 volts. While the vehicle is running the battery that was reading 12.9 volts jumps to closer to 16 volts, while the other battery only jumps to about 11.6 volts. When checking them together without the vehicle running, the batteries are reading 24 volts, and when running, ready pretty close to 28 volts.

Now I am all new to 24v systems, so I may be going about this completely wrong. I am trying to figure out why the power fluctuates on initial start up after sitting for a day or 2. Am I going about this the right way with my diagnosis?

Any input would be greatly appreciated. Thanks!

 

[Coffey1]

What I was told glow plugs cycling

 

[Coug]

The glow plugs cycle in short bursts when the engine is first turned on. The needle will bounce around because the generator is lagging just a little behind the glow plugs for power output, so voltage dips briefly, then it spikes as the plugs draw power then stop.
If one of your batteries is failing or not charged, the needle bounce will be worse. I ran mine for a couple months on an old pair of 650CCA batteries. They started the truck no problem, but the needle would dip down to the red, or spike up to the red near the end of the plugs cycling.
That part is perfectly normal (provided it stops after a couple minutes)

If one batter is much higher voltage than the other one then you have a failing battery. The generator is going to put out the 28V during operation, and it doesn't care how it's distributed to the batteries. The fact that one is seeing 16 and the other under 12 tells me that it's very likely that low battery has a shorted cell.

If you have a 12V battery charger disconnect the batteries and charge the low one separately. If it comes up to over 12.6V overnight then maybe the issue is somewhere else, but I'd be willing to bet that it's a failed battery.

 

[Coug]

oh, and continuing to run it in this condition where one battery is getting 16V is very very bad for that battery. Not sure if the optimas can explode like a flooded lead acid, but it will definitely shorten it's life.


Btw, it would also help for use to know what generator is in the truck, and if it's the 3 or 4 speed, as well as any accessories you might have added, especially if they are only wired to one battery for 12V

 

[radrob]

The glow plugs cycle in short bursts when the engine is first turned on. The needle will bounce around because the generator is lagging just a little behind the glow plugs for power output, so voltage dips briefly, then it spikes as the plugs draw power then stop.
If one of your batteries is failing or not charged, the needle bounce will be worse. I ran mine for a couple months on an old pair of 650CCA batteries. They started the truck no problem, but the needle would dip down to the red, or spike up to the red near the end of the plugs cycling.
That part is perfectly normal (provided it stops after a couple minutes)

If one batter is much higher voltage than the other one then you have a failing battery. The generator is going to put out the 28V during operation, and it doesn't care how it's distributed to the batteries. The fact that one is seeing 16 and the other under 12 tells me that it's very likely that low battery has a shorted cell.

If you have a 12V battery charger disconnect the batteries and charge the low one separately. If it comes up to over 12.6V overnight then maybe the issue is somewhere else, but I'd be willing to bet that it's a failed battery.

Thanks for that info. I had a sneaking suspicion it could be a failed battery, but wasn't too sure. Fortunately I bought 4 of them, so I have 2 backups if I need to swap one out.

I did leave out a little bit of information that after a little more digging might be another contributing factor here. I have a power inverter hooked up to one of the batteries to supply power to some accessories I am using during testing. Its a 12v inverter, and I only have it hooked up to one battery, which happens to be the battery that is reading low on voltage. After some discussion today, I am wondering if running that inverter off the one battery is what is causing the imbalance as it may be draining just that one battery alone, but I don't really know if that makes sense. We've discussed removing it and replacing it with a 24v inverter instead, but part of me thinks it may just simply be a failing battery, but I thought it might be worth noting, as that may be something that could cause it to read lower than the other battery. Keep in mind, all of the diagnosing I was running earlier was all with the inverter off, as I haven't had to use it in a few weeks.

As for the fluctuating gauge and headlight dimming, that makes sense what you are saying. The fluctuation is pretty minor as well as the headlights, so I'm assuming what you are describing is exactly what I am experiencing so I won't worry too much about that.

I'm going to pull the lower voltage battery this afternoon anyways and throw it on the charger and go from there.

 

[Coug]

If you have the 60 amp generator in your truck running 12V anything from a single battery is a big no-no.
The earlier trucks with 3 speed and 60 amp generators only supply 24V to the batteries together, it doesn't do anything to make sure that each battery is getting an equal amount. Usually the front battery ends up charging a little more than the rearward battery, so swapping their positions every month or three is recommended to keep them charging evenly.

Unless you have the 200 amp generator system in your truck, which has a 12V charging circuit built in, you DO NOT want to hook anything to just one battery. It's almost 100% guarantee that the inverter is your issue here, and is responsible for killing that battery. Either go 24V inverter or upgrade to the 200 amp charging system.

 

[radrob]

If you have the 60 amp generator in your truck running 12V anything from a single battery is a big no-no.
The earlier trucks with 3 speed and 60 amp generators only supply 24V to the batteries together, it doesn't do anything to make sure that each battery is getting an equal amount. Usually the front battery ends up charging a little more than the rearward battery, so swapping their positions every month or three is recommended to keep them charging evenly.

Unless you have the 200 amp generator system in your truck, which has a 12V charging circuit built in, you DO NOT want to hook anything to just one battery. It's almost 100% guarantee that the inverter is your issue here, and is responsible for killing that battery. Either go 24V inverter or upgrade to the 200 amp charging system.

Yup, I think you are absolutely right. That is the configuration we have in ours, so I will definitely remove that power inverter immediately. I fortunately didn't use it a whole lot, so I may still be able to bring the battery back, but I will definitely swap them back and forth every few months like you stated.

Thanks a ton for your help and info, it was a massive help!

 

[papakb]

There are a couple of things going on here. It would help to know which of the alternator/regulator systems you have in your truck because diagnosing problems changes slightly between them.

Military gauges are built by the lowest bidder and aren't always of stellar quality. I've had new gauges that will all read slightly different so keep that in mind.

The 60 amp alternators are notorious for wandering voltages. This happens when they are starting to fail or have already failed. Worn brushes, bad regulators or blows resistors can all show up this way.

Corroded battery terminals can cause voltage reading to vary between batteries. Always put your probe on the battery post and not the terminal lug.

You can read the actual system battery voltage safely on pins E(+) and F (-) on the STE/ICE connector beside the shifter.

If the engine temperature is below 120 degrees the glow plugs will cycle until it warms up. Depending on how cold it is this can take a few minutes to happen.

As stated above, never tap 12 volts off of one battery. It's bad practice and if done wrong can cause severe electrical issues. Your better off buying a 24 to 12 volt converter and allowing your equipment to draw power from both batteries. Just make sure the converter you buy is capable of supplying enough power to the equipment you connect to it. Good engineering usually wants a 50% safety margin so if you have a 10 amp component you want a 15 amp converter.

When using inverters your calculations are all based on power as much as voltage since power conversion is what they're doing. The current drawn into the inverter is based on how much it uses to increase the voltage plus the current drawn by the equipment itself. A 1000 watt inverter needs around 80 amps of 12 volt power. Inputting 24 volts drops that rating by half, or around 40 amps so size your input wiring appropriately.

 

[Smckerigan]

Mine 94 was doing the same thing so I replaced the voltage regulator last week and now it stays right on the mark.

 

[Mogman]

I would like to add one more thing here, common knowledge but for the benefit of the newbie/newbies
24V systems are a completely different animal compaired to 12V systems, ALL CONNECTIONS AND ESPECIALLY BATTERY TERMINALS SHOULD ALWAYS BE KEPT CLEAN AND TIGHT!!! do NOT skimp here or just "temporarily" connect the battery by stabbing the connector onto the battery, it can and has many times toasted the terminals completely off of the batteries. I have seen this more than once and the best way to describe it is like a huge cutting torch erupting ontop of the batteries.