Installing 12 volt accessories into 24 volt systems

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papakb

Well-known member
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San Jose, Ca
Installing 12v radios into 24v system is filled with hazards. People who connect their radios across the high side battery wonder why they smoke when they hook up the antennas. Here's the key. In a HMMWV or any 24 volt system you have 2, 12v batteries connected in series to give you 24 volts. The "low side" battery is the one with it's negative terminal connected to the current shunt in the battery box. This is effectively the ground point in the HMMWV. It's that funny looking brass thing that looks like a cheese slicer in the left front of the battery box. In a regular MV this point would be connected to the vehicles frame. The HMMWV doesn't rely on the body as the ground because of the way it's build but rather includes ground wires in the electrical harness to perform this function. The positive post on the "low side" battery is the one that connects to the inter battery jumper, the short link between the 2 batteries. This then connects to the negative terminal of the "high side" battery. The voltage at this point is +12 volts with reference to the current shunt. The positive terminal on the "high side" battery is the point where the electrical system gets its +24 volts. The heavy wire that attached here runs directly to the + terminal on the starter motor in the early series trucks and to a large brass terminal block in later trucks. This terminal strip is heavy brass strip with numerous bolts on it that distributes that +24 volts out to the system.

When you accidentally connect things across the "high side" battery you will get +12 volts from the battery but when you connect the accessory negative lead to the negative post that point is still 12 volts above the vehicles ground and if the accessories case or antenna touches the body current will flow and things will burn up. Connecting accessories that draw lots of power, even when connected properly across the "low side" battery causes an imbalance when the batteries charge that unevenly wears out your batteries.

If you want to run 12 volt accessories in your truck your best bet is to install a voltage dropper (voltage regulator) that connects across the system +24 volts and electronically regulates it down to +12 volts. Connecting your 12 volt accessoreis across the regulators output automatically connects the ground to the right point in the electrical system and draws power from both batteries so they will charge normally. The thing you need to be careful with here is that you buy a dropper (regulator) that can provide enough current to power your device. If it's something like a simple radio drawing 1 or 2 amps buy a regulator that will provide 5 amps. This builds in a safety cushion. Sometimes people will incorrectly refer to this as a transformer but transformers do not work on the DC current that batteries provide.

I hope this simple explanation helps.
 

rmesgt

Active member
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Grove, Oklahoma
I would like to install a radio/CD player in my M1009. I thought this was going to be simple, but after reading your post I can see that I could have made a HUGE mistake. The step down regulator seems to make the most sense, but I don't know where to get one. I imagine that when I find one, it will provide installation instructions. Do you know a make/model of this type of device? Please advise
 

Valor

Member
61
23
8
Location
Apple Valley, Ca.
Google is your friend. Try searching for Daygreen B20-24-12. That's a 24 volt to 12 volt DC downconverter that can handle 20 amps. That is overkill for a radio but good for extra accessories too. There are 24 volt to 13.8 volt DC downconverters too. You can get them in various amp ratings. Easy hook up. 2 wires to your 24 volt batteries and 2 wires 12 volts output to your accessories. Easy peazy!
 

155mm

Chief and Indian
Steel Soldiers Supporter
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Location
Guymon, OK
I had a 998 that had major electrical issues, burn up alternators, lights not working erratically, ste ice not functioning correctly, so it got sent to me. It was an ecod from sliding on the ice into a telephone pole, ended up being that ground shut had broken.... just passing on my knowledge in that department
 

richingalveston

Well-known member
Steel Soldiers Supporter
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Location
galveston/Texas
A 1009 has a frame ground and no shunt. you can connect to the 12 volt battery and not have any of the above issues except the battery imbalance that is normally not a big issue unless you are using a lot of 12 volt power. if you swap you batteries every 4 to 6 months it will help in keeping the batteries aging the same.
 

Mikei998

New member
16
0
1
Location
Lewiston NY
Installing 12v radios into 24v system is filled with hazards. People who connect their radios across the high side battery wonder why they smoke when they hook up the antennas. Here's the key. In a HMMWV or any 24 volt system you have 2, 12v batteries connected in series to give you 24 volts. The "low side" battery is the one with it's negative terminal connected to the current shunt in the battery box. This is effectively the ground point in the HMMWV. It's that funny looking brass thing that looks like a cheese slicer in the left front of the battery box. In a regular MV this point would be connected to the vehicles frame. The HMMWV doesn't rely on the body as the ground because of the way it's build but rather includes ground wires in the electrical harness to perform this function. The positive post on the "low side" battery is the one that connects to the inter battery jumper, the short link between the 2 batteries. This then connects to the negative terminal of the "high side" battery. The voltage at this point is +12 volts with reference to the current shunt. The positive terminal on the "high side" battery is the point where the electrical system gets its +24 volts. The heavy wire that attached here runs directly to the + terminal on the starter motor in the early series trucks and to a large brass terminal block in later trucks. This terminal strip is heavy brass strip with numerous bolts on it that distributes that +24 volts out to the system.

When you accidentally connect things across the "high side" battery you will get +12 volts from the battery but when you connect the accessory negative lead to the negative post that point is still 12 volts above the vehicles ground and if the accessories case or antenna touches the body current will flow and things will burn up. Connecting accessories that draw lots of power, even when connected properly across the "low side" battery causes an imbalance when the batteries charge that unevenly wears out your batteries.

If you want to run 12 volt accessories in your truck your best bet is to install a voltage dropper (voltage regulator) that connects across the system +24 volts and electronically regulates it down to +12 volts. Connecting your 12 volt accessoreis across the regulators output automatically connects the ground to the right point in the electrical system and draws power from both batteries so they will charge normally. The thing you need to be careful with here is that you buy a dropper (regulator) that can provide enough current to power your device. If it's something like a simple radio drawing 1 or 2 amps buy a regulator that will provide 5 amps. This builds in a safety cushion. Sometimes people will incorrectly refer to this as a transformer but transformers do not work on the DC current that batteries provide.

I hope this simple explanation helps.
 

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Mikei998

New member
16
0
1
Location
Lewiston NY
You only need the box in the pic or you can install the optional Solar . Got mine on eBay . Allows you to hookup 12 v and 24 v fuse box to dual battery’s. Works great !
 
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