papakb
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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.
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.
