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need reliable 12-13 volts from 24volt wheelchair

I have a 24 volt wheelchair that runs on 2 12 volt batteries in series. I need output not to exceed 13 volts for my cpap machine. There are many DC-DC converters that are labeled 24 to 12 but it seems all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly stated that any voltage above 13 may damage the equipment. Is there a way to reliably shave a volt or so off of the converter output? Perhaps by using a diode in series? The Cpap is rated at 40watts max or 3 amps at 13 volts. Any suggestions are welcomed, the more specific the better. Thanks

Reply to
dpondoc
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The AM 1301-7R is rated 12 volts at up to 4 amps out with 8 to 35 volts in.

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They run about $500, though.

Digikey shows lots of other choices, but most, including this one are not in stock. But their search choices can give you some part numbers to look for. If you want good reliability, pick a unit that is rated for a bit more than you need. 50 to 100 watts is possibly a good max rating for a 40 watt load.

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Regards,

John Popelish
Reply to
John Popelish

I have a couple of questions. Why do you have to use the wheelchair batteries to power your cpap machine? It would make more sense to have a 12 volt golf cart or forklift battery sitting by your bed, dedicated to cpap use. Charge it during the day. How do you power your cpap machine now? You would need a good reason to use a battery bank that has an inappropriate voltage and then down-convert it again, with the added complexity and losses (and expense) that entails. If you power the cpap machine directly from a suitable battery you run almost no risk of a sudden power failure. Add a dc-dc converter to the setup and you increase the risk. A dc-dc converter would make sense for use away from home. But if you stay in hotels, then using a 12 volt power supply that simply plugs into the wall would cost a lot less and wouldn't draw down your wheelchair batteries at night. Just throwing out alternative ideas. But perhaps you have your own reasons for wanting the converter, like simply taking naps in your wheelchair, in which case plenty of engineers on this board can advise you. I see John P has already come up with a suggestion.

Reply to
gearhead

Is there some reason why you can't simply take a tap from the center of the two battery=20 connection?

Reply to
Greg Neill

The batteries are generally charged in series. It wouldn't take long to destroy the pair.

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Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

On Fri, 29 Feb 2008 07:36:21 -0800 (PST) in sci.electronics.basics, snipped-for-privacy@gmail.com wrote,

Perhaps an internal adjustment?

Reply to
David Harmon

Stick some diodes each with a voltage drop of around 0.7 volts in series with the output of the 13.7 volt battery?

# Diode Voltage Diodes Volt drop. Net from 13.5 to 13.8 v

0 0 13.5 to 13.8 1 0.7 12.8 to 13.1 2 1.4 12.1 to 12.4 3 2.1 11.4 to 11.7
Reply to
terryS

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