# 12v from 24v wheelchair

1. Apr 8, 2004

### vars

Here is the situation: I have a wheelchair powered by two 12v bridged batteries (24v). I like to have 12v output to power a laptop computer. I estimated that I need less than 4A. I do have access to batteries and I think that I do not need to use very expensive 24v-12v converter. It is a way to build circuit using for example diodes or something like this to achieve 12v output.

Thanks for any input.

2. Apr 8, 2004

### dduardo

Staff Emeritus
The 7812 voltage regulator only handles 1A so you'll need one that can handle more current. Here is a place you can get one:

http://www.linear.com/prod/datasheet.html?datasheet=620 [Broken]

Last edited by a moderator: May 1, 2017
3. Apr 8, 2004

### Cliff_J

Simply tap off just one battery at a time. This is, assuming of course, that you are using a car adaptor to run the laptop. If you load balance between the two, the effect should be minimal.

BTW, the best way to refer to how the batteries are connected is "in series with each other" instead of 'bridged' which has other meanings.

To use a linear regulator to drop that much voltage and with that much current will require a large heatsink, and will double the wattage you're pulling from the batteries that will shorten the charge life of both of them.

If the current requirements are steady (which they should be) you can use resistors instead of a linear regulator. That would be the lowest cost solution, and you should find a 100W resistor and make sure you find out if it needs a heatsink. Again you'll be pulling twice the power you would had you just tapped one of the batteries.

Cliff

Last edited: Apr 8, 2004
4. Apr 8, 2004

### vars

Somebody give me this schema. Is it correctly?

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Last edited: Apr 8, 2004
5. Apr 8, 2004

### Cliff_J

vars, in that diagram the left diode would heat up and burn out after just a few seconds (depending on how big it is). Then you would just be using the left battery (or what's left of it after it worked on burning up the diode).

You'd be better off using a DPDT switch to connect your laptop adapter to one or the other battery. Again, I'm assuming you're using a car adaptor in-between the battery and the laptop, a 12V battery might be 13.8V (like the powered wheelchair I've tested) at full charge and that might be too much for a laptop straight up without an adaptor. The automotive laptop adaptor should provide the correct voltage to the laptop, regulated and filtered as much as should be needed to meet the power requirements of the laptop, otherwise it might not charge the internal battery properly or cause damage to the unit. Everything has a safe operating range, and you don't want to exceed it by trial and error.

Cliff

6. Apr 8, 2004

### wimms

not seconds, but microseconds, with bang, explosion and lots of smoke.
Just tap one battery at a time..

7. Apr 8, 2004

### dduardo

Staff Emeritus
Ah, now I understand whats going on. But I have a question for you vars. Why do you need to hook up the laptop to the wheel chair battery? Why don't you just run the laptop off its own battery? Seems a little redundent to me.

8. Apr 8, 2004

### Cliff_J

Here's a quick diagram of how to wire up a DPDT to use just one battery at a time. Try to find a DPDT that has on-off-on settings, that way you could disconnect power to the laptop altogether.

Last edited by a moderator: Mar 4, 2015
9. Apr 8, 2004

### Jake

Hmm, couldn't you just connect the batteries in parrallel?

10. Apr 8, 2004

### vars

Quick update

First of all, thank you very much for your input.

(ddurado) Sometimes I'm long hours outside and original batteries are dying too fast, so I like to have an option to connect my devices somewhere and I thought that the wheelchair batteries may be an option.

(Cliff_J) Yes I'm planning to use car adaptor.

11. Apr 9, 2004

### Cliff_J

wimms - depends on the diode one from an old alternator could last much much longer than a 1N4007!

Jake - that might work ok for the laptop but not the wheelchair!

Vars - good luck. Like I said, just make sure you keep the load-balancing in check (use one 10 minutes, then the other 10 minutes type thing). The wheelchair batteries I've seen are lead-acid and you want to make sure that one doesn't get discharged more than the other or the useful life of the battery will be shorter. Lead-acid batteries generally like to be fully-charged all the time, anything else is shortening the useful lifetime of them.

Cliff