The Chairman's Lesson in Electrical 101
[Intro from the Psychic] I was looking at some specialty lighting for my golf cart for an upcoming parade, and saw that it needed 12 volts to operate. They sold a separate 12volt, rechargable battery, but I wasn't sure if I need it or if I could simply tie into the golf cart's battery series. I posed the question to the resident expert about such matters at Psychic Golf Cart Repair, the Chairman, and he provided the following information.
[Begin info from the Chairman]Ok! You already have battery power so you don't need another battery to have to fool with. Those little batteries would have a very short charge / duty cycle. They are used mostly for backup power on bugler alarms when the power goes out. However, you might get an hour out of one if the amp draw is really low. They are usually rated in mHr which is Milli Amps and not Amp Hours. That's a 100th of an amp.
Here's how to calculate volts, amps and watts using Ohm's law.
(Volts) X (Amps) = Watts
(Watts) / (Amps) = Volts
(Watts) / (Volts) = Amps
In a series circuit like a battery connected + to  to + to  etc, the voltage adds but you do not increase the amp draw capability compared to the amp rating of each battery. It stays constant. But you do raise the voltage which increases the wattage output because (Volts) X (Amps) = Watts. This is how a high draw system like a golf cart can run for several hours. If the battery pack is wired in Parallel like + to + to + etc, the voltage stays the same but you increase the amperage. So, if you had 4  12volt batteries in parallel the voltage would be 12 but the amps multiply. Actually, in parallel you're just simply building a larger battery based on the same voltage per battery with much larger amp capacity. Just remember In series you’re adding volts but no more amperage.
So if you know the amp draw of the load or just know the resistance then you can calculate amp draw. If the resistance measured in ohms (could be a decimal ) you would take (Volt) /

This is Georg Ohm.
Not the Chairman.

Resistance (ohms) = Amps. A load measuring 2 ohms on a 36volt series system would be (36) / (2) = 18 amps which is the amp load on the pack. A 400 Amp Hour deep cycle battery pack could sustain this load for about 22 hours. 400 / 18 = 22 hours. But, in real life it's usually less because the draw curve is not straight and more like a reverse parabolic (curves down).
We now know batteries set in series will add voltage when measured on the two end poles of the battery pack + and  so 2, 12 volt batteries will give back 24 volts. BUT! If you tap just one battery you'll only get the voltage of that particular battery. So, in your case where you need a 12 volt source, you just find 2, 6 volt batteries set in series and tap the + and  terminals at the ends. This way you have 12 volts and can pull off them for a pretty long time. However, your amp draw is cumulative so if the system on 12 volts is drawing 5 amps and the golf cart is drawing 25 amps this equals a total of 30 amps. If your battery capacity is say 400 amp hours at 25 amps this means you can draw a steady 25 amps for about 61/2 hours. But, a golf cart draws a huge amount of amps on start up as from a standing start which could reach well over 100 amps due to the natural physical properties needed to accelerate an object. More weight = more resistance, hills = more resistance and so forth. This is why a typical golf cart can only go for about 4 hours before the seeing the power drop off. It may still run okay, but performance is going down. Now, this does not necessarily mean the voltage is drastically dropping because the voltage is really staying fairly constant but the amps are falling away. So a golf cart with a 36volt system may be fully charged and have 38 to 40 volts on the battery pack. But when at a state of discharged might measure 35.7 volts or so. You’ve only lost a few volts but the amps are used up. Its ability to work is rated on the amps available and a surge of amp draw will pull the pack down to a very low amp measurement. A fully charged pack may read just a little over the cumulative voltage of the pack but the amp capacity is has increased dramatically.
So my young Padawan, we will reflect on this lesson and contemplate its meaning. May the Force with you!
The Chairman
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