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Electrical question


Restorer32
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We are restoring an early electric car. It is 84 volts and originally used 90 volt bulbs in the headlights, side lights, even dome lights. We want to convert these lights to 12 volts in the interest of safety and convenient maintenance. Now for the question for the electrical engineers in the crowd. We will be running this car using 7 12 volt batteries in series to produce 84 volts. Can  we simply  tap off 1 of these batteries to get 12 volts?  I know we can with the motor not running but how about when the motor is running? Will the 84 volts back feed thru our tap ? I know my way around electric circuits but I am not 100% sure about this one.

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No, will not back feed . Just tap any battery in the set . On golf carts they tap two series six volt to give you the twelve for accessories . Diagrams available on line to verify .

  Only difference with motor running o is voltage dropping a little as with it going up a little during charging . Neither should matters . Make sure to fuse . and fuse both if using center battery for tap . This is just incase wire gets pinch grounded . It will open . Fuse close to tap .

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I think I would run a gel cell motorcycle battery for the 12 volt lights, just put a battery tender plug on it for convenient charging. You can probably hide the small battery easier than running wires under the front or rear hood to pick up the 12 volts off the driving batteries. Also, if the lights get left on for some reason, it won’t kill the main drive batteries. Are you running lead acid or “glass mat” batteries? If you change led acid batteries, you’ll have to open everything up to prevent hydrogen gas buildup and the acid fumes attacking the paint and finish. I would run Optimas ........they should last for ten years without issue.

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29 minutes ago, ArticiferTom said:

No, will not back feed . Just tap any battery in the set . On golf carts they tap two series six volt to give you the twelve for accessories . Diagrams available on line to verify .

  Only difference with motor running o is voltage dropping a little as with it going up a little during charging . Neither should matters . Make sure to fuse . and fuse both if using center battery for tap . This is just incase wire gets pinch grounded . It will open . Fuse close to tap .

 

^^^This. It's no different than when we no-good hot-rodders use two six volt batteries in series to convert an old car to 12V but still want to have 6V available for the original instruments or accessories.  😁

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The problem with tapping into one of the 12 volt batteries is that this will drain that battery more than the other 6 (edited because I can't add). Best to use LED globes in the headlights to minimize draw from this battery.

You have two other options,

1. As has been mentioned, an accessory stand alone 12 volt battery, charged separately.

2. DC-DC charger to keep the accessory  battery charged from the main pack.

Edited by maok (see edit history)
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Maok is right on the money about draining the 1 battery at a different rate then the other 7. My starter/generator test stand uses 2 6v batteries and I am always having to monitor them and charge them separately to keep them balanced. I would go with a single small motorcycle or lawn tractor battery with a separate charger.

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I would drive it at night if I owned it, but then again I drive everything I own, to hell with looking at trailer queens. Restore them to 100 point perfection and then consume them. You can always go back and restore it again.

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This Rauch and Lang electric is going to a customer of ours who lives in Paris. He intends to use it at his summer home in Normandy, which conveniently has a carriage house. We also restored an original charger/rectifier for him to display. We intend to show it next month at that big car show in PA.  

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