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Two 12v batteries won't give you 24v. Your Hummer already has 2 12v batteries in it and it's still a 12v system right?
You are correct, two batteries can provide both 24 Volts (nominal), or 12 Volts (nominal), depending on how they are wired.
If the positive terminals are connected together (same for the negative terminals, which are connected to ground, chassis, in modern vehicles), the system is a 12 Volt system.
If one battery has its positive terminal connected to the negative terminal on the other, it is a 24 Volt system. Of course, of the two remaining terminals, the "free" negative terminal is grounded (connected to the chassis), and the "free" positive one is connected to the rest of the electrical system.
IF the batteries are the same type, the POWER available from both 24 V and 12 V systems are exactly the same. The current available is double in a 12 V system, and the wire size has to be larger in order to carry the higher current with the same loss of power.
[Current flowing through a non perfect conductor creates heat, and the amount of heat varies depending on the resistance in the wire. The higher the resistance OR the current, the more heat, and the more power lost. If the heat gets to be too great, the wire will burn.]
Hummers are 12 Volt systems. This makes them compatible with the other vehicles on the road. Then why do we need two batteries? In order to power the glow plugs, and crank the high compression diesel engine. I don't know for sure, because I no experience with gas Hummers, but my guess is they have only one battery. It does not take two to crank a 350 small block.
Humvees are 24 Volt systems, making them compatible with MOST of the military vehicles in service. The military is trying to standardize on equipment, for ease of fueling, and maintenance issues like jump starting, and battery charging. The Army has settled on 24 Volts, and JP-8 for fuel. JP-8 runs their helicopters, and will work just fine in diesel engines (a bit less power, but MUCH cleaner). Sure, they have some legacy systems left that use gasoline, and 12 Volt starter/electrical systems, but they are being retired.
To further confuse the civilian issue, diesels have historically used 24 Volt systems. This is because of the higher compression. While it is true that two batteries, whether parallel (12 Volts), or series (24 Volts) wired will produce the same power, it is MUCH easier to transfer power at higher voltages. The brushes and contractors suffer much less abuse if the CURRENT is lower. [This is one reason the power companies use high tension (high voltage) transmission lines.] The other reason 24 Volts is used is because the wires can be smaller. Smaller wires as battery cables is good, but it make motor design and construction much easier, like the starter motor.
However, especially in light trucks, 24 Volts was just too much trouble for the owners and users. Radios need special attention, through the use of a second alternator, or a special switching power supply to provide them with 12 Volts. Bulbs are special, somewhat hard to find, and more expensive. Further, bulbs are the ONE CASE where higher current is better. The filament has to be thicker to produce the same light, so they are physically stronger and will take more vibration before breaking.
Still, most large trucks, buses, and even some RVs use 24 Volt systems.
I have yet to wade in on the jump starting issue. Here goes. ONLY JUMP START 12 VOLT SYSTEMS FROM 12 VOLT SOURCES, AND ONLY JUMP START 24 VOLT SYSTEMS FROM 24 VOLT SOURCES (with one POSSIBLE exception).
If you have a dead 24 Volt vehicle, it is very unlikely 12 Volts would be any help. Also, since batteries are "dead" at about 9-10 Volts, you will be further draining the batteries, drawing 18-20 Volts down to 12. Someone suggested just jumping to one (hopefully the grounded) battery. If the 24 Volt system is not really dead, but marginal, this is safe to people, and not too rough on the batteries.
Also, keep in mind that if you are actually successful starting a 24 Volt system from a 12 Volt system, as soon as the 24 Volt system starts, the alternator will product 24 Volts, and through the jumper cables, it is going to force 24 Volts into the 12 Volt vehicle. Good-bye stereo. Good-bye bulbs. Maybe good-bye voltage regulator, and alternator. Leave 24 Volts across the 12 Volt battery for any length of time and you can have it explode.
If you have a dead 12 Volt vehicle, don't even think about jumping it from a 24 Volt system. Will you be able to start the vehicle? Sure. Will you burn up, or shorten the life of electrical components in the 12 Volt vehicle? YOU BET!
Would you normally even think about hooking your car stereo up to 24 Volts? Well, you are, even if it is off. The days of a "power switch" actually turning off the power to a component are LONG over. Your CD player, your head, your amps, ALL have power applied to the circuitry, and are held "idle" by the power management circuits. Also, the clock is always powered. Also, as above, you risk a battery explosion, and damage to the voltage regulator, the alternator, and just about anything else electrical.
Any bulbs which are on will have twice the Voltage applied to them. Two times does not seem that bad does it? Well, because the Current is a direct result of the applied Voltage and the Resistance, the Current doubles too! This means the bulbs consume FOUR TIMES the power -- though for a short time.
The exception? If you really know what you are doing, you can jump from the middle of the battery stack. Hook the positive jumper cable to where the two batteries are connected together (+ to -) and connect the ground lead to ground. This will give you 12 Volts for jumping. A warning though, it is not good for the batteries, though it should not cause a problem if done only very occasionally. Much worse is when people tie constant loads, like stereos, to that point to get 12 Volts. The high battery is constantly overcharged, which will boil out the water. The grounded battery will be constantly undercharged, leading to a very short life.
There are 24 Volt battery chargers. If one buys two of the "proper" 12 Volt chargers, they can be sued to charge a 24 Volt system, if wired in series. The "proper" charger must not have the AC line ground pin connected to the ground (black) charging lead.
If the AC line cord ground pin is connected to the "black" or minus lead, the batteries will have to be disconnected in order to charge them at the same time.
Also, you could disconnect the series wired batteries and temporarily wire them in parallel for charging from one 12 Volt charger.
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