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Login to post Forums » Off-topic discussion » Help me understand batteries

Dec. 5, 2009 3:18 p.m. SVreX SuperDork

I've got a bit of a strange application.

When my mother moved into her home 2 years ago, we recognized she needed a wheelchair lift. Wheechair lifts are expensive, so in typical grassroots fashion I came up with an alternative.

We found an ebay bargain on a wheelchair lift designed for a bus. It was well made, and it's hydraulics were pretty simple, but the electrical was convoluted. It was designed to run off dual controls at both the top and bottom, with an over-ride at the driver's seat. We got all the controls, etc.

It also ran on 24V.

I modified the harness to work, and did a lot of searching for an converter that would convert the 120 AC household current to 24V DC. That turned out to be a rare animal.

So, the alternative I chose was to install 2 deep cycle automotive batteries in series. Worked great.

The batteries would last about a week before needing a re-charge. Ultimately, I found 2 small 2A 12V trickle chargers and hooked one to each battery (with the batteries still connected to each other). I don't know why this seemed to work- my logic was saying that 12 volts tied to 2 different sides of a 24 volt system was not going to work, but it did work fine for 2 years.

But this week both of the chargers died. The batteries are fine.

So, my question is, was this just a screwy setup that was doomed to fail and I got what I deserved, or is it just a coincidence that they both died together? Should I just go buy 2 new chargers, or do I need to make an effort to find the 24V charger or converter? Why did it work in the first place?

Just looking for a bit of an education.
Dec. 5, 2009 3:44 p.m. TJ Dork

I think I have an old 24 VDC charger. It was for my son's old power wheelchair. I think I have it stowed away out in the garage somewhere - if I can find it, you can have for the cost of shipping.
Dec. 5, 2009 3:47 p.m. TJ Dork

If you hooked up each of the two 12 V chargers to each of the batteries separately then that should've worked. Just don't hook up a 12 V charger across all 24V.
Dec. 5, 2009 4:05 p.m. MrJoshua SuperDork

Yup, cheapie chargers died a natural death. Your wiring was fine.
Dec. 5, 2009 10:03 p.m. mad_machine SuperDork

bought at the same time, used the same amount of time.. wore out at the same time.. Sounds about right
Dec. 5, 2009 10:33 p.m. DrBoost HalfDork

No, if you hooked up 2 12V chargers across 24V you will fry something, and if they are cheap chargers, that's what'll go. I can't find a link right now, but there are switches that will allow you to throw a switch (similar to a batt d'connect for a race car) that will basically switch the cables between the batteries, making them 2 12V batteries when you charge, then switch them back to 1 24V battery. Just make sure you keep track of the switch position.
Dec. 6, 2009 7:31 a.m. SVreX SuperDork

SVreX wrote:

Ultimately, I found 2 small 2A 12V trickle chargers and hooked one to each battery (with the batteries still connected to each other). I don't know why this seemed to work- my logic was saying that 12 volts tied to 2 different sides of a 24 volt system was not going to work,

I think some of you missed the highlighted part. They were hooked up when the batteries were connected to each other.

It was 24 volts.
Dec. 6, 2009 7:32 a.m. SVreX SuperDork

TJ wrote:

I think I have an old 24 VDC charger. It was for my son's old power wheelchair. I think I have it stowed away out in the garage somewhere - if I can find it, you can have for the cost of shipping.

That would be awesome. Let me know.
Dec. 6, 2009 8:53 a.m. Dr. Hess SuperDork
I can see the 2 12V chargers working. I assume you hooked it:
- battery1 - + battery2 -
with the chargers across each battery individually. I think it also might have worked if you did it with the + of charger 1 hooked to + of battery 1, - of charger 1 hooked to + of charger 2, - of charger 2 hooked to - of battery 2. Depending on the charger types you use, I think that should give you a floating ground in the middle and charge the batteries. A proper 24V charger would be better, of course. And if one charger died, it could get messy. Some fuses in there would be a good idea.
Dec. 6, 2009 10:46 a.m. MrJoshua SuperDork

SVreX wrote:

SVreX wrote:

Ultimately, I found 2 small 2A 12V trickle chargers and hooked one to each battery (with the batteries still connected to each other). I don't know why this seemed to work- my logic was saying that 12 volts tied to 2 different sides of a 24 volt system was not going to work,

I think some of you missed the highlighted part. They were hooked up when the batteries were connected to each other.

It was 24 volts.

But they were still hooked up to the + and - of each individual battery right?
Dec. 6, 2009 11:33 a.m. stuart in mn SuperDork

24v chargers are pretty available. Here's one that looks like it would do the trick. http://www.batterystuff.com/battery-chargers/24-volt/multi-volt-input/JAC0891-34.html
Dec. 6, 2009 11:43 a.m. alex Dork
Dr. Hess wrote:

I can see the 2 12V chargers working. I assume you hooked it:
- battery1 - + battery2 -
with the chargers across each battery individually. I think it also might have worked if you did it with the + of charger 1 hooked to + of battery 1, - of charger 1 hooked to + of charger 2, - of charger 2 hooked to - of battery 2. Depending on the charger types you use, I think that should give you a floating ground in the middle and charge the batteries. A proper 24V charger would be better, of course. And if one charger died, it could get messy. Some fuses in there would be a good idea.
The Doc's on to something here.
Dec. 6, 2009 1:28 p.m. Dr. Hess SuperDork

I would strive to use chargers with only 2 wire 110V plugs, not 3 wire if you're going to try my idea. You never know if that third wire is connected to the battery lead negative or not, and "results may be highly unpredictable" otherwise. You could try 3 wire 110V side chargers, but use one of those old 3wire->2 wire plug things and make sure the 3rd lead (ground) isn't grounded. A 10 amp fuse in series with one of the battery lines should be plenty.
Dec. 6, 2009 3:59 p.m. SVreX SuperDork
Doc's first guess was correct:
Dr. Hess wrote:

I can see the 2 12V chargers working. I assume you hooked it:
- battery1 - + battery2 -
with the chargers across each battery individually.
I can't even BEGIN to follow you with that other idea.
Dec. 6, 2009 4:29 p.m. Dr. Hess SuperDork

I dunno how else to simplify my other suggestion, Rex. How about: You have two bateries hooked in series. The + on one goes to the + on your motor. The - on the other goes to the - on your motor. Somewhere in there is a control circuit, which I'll ignore. Try taking two battery chargers. The plus on charger 1 goes to the + on your battery that goes to the motor. The - on charger 1 goes to the + on charger 2. The - on charger 2 goes to the minus on the batteries that goes to the motor. Put a fuse in there somewhere.

Or buy a 24V battery charger.
Dec. 6, 2009 5:48 p.m. mad_machine SuperDork

I can see the gist of what Hess is saying. And it would work. It is essentially making two 12 volt chargers act like a 24 volt one.. in the same manner than two 12 volt batteries hooked in a series makes a 24 volt battery.

And yes.. if one charger dies.. it will get VERY messy quickly. I am also willing to reconsider my earlier thoughts on both chragers dying because they were the same age and used the same amount of time. I am now willing to say that one charger died and the resulting load from trying to charge two batteries did in the second

This is twice in one week the Good Doctor and I have agreed on something... is this allowed or part of the coming apocolypse?
Dec. 6, 2009 6:23 p.m. xci_ed6 Reader

the neutral 110v input is still a reference to ground, I think connecting the chargers in series is a recipe for disaster. The trickle chargers probably have a solid state voltage regulator that would still be regulating to 12v. It would fry one.

The fact that your other setup lasted two years, and not less than 5 minutes makes me think it was working fine. I'd go get a couple $5 battery maintainers from harbor freight and do it again.
Dec. 6, 2009 8:34 p.m. SVreX SuperDork

Dr. Hess wrote:

I dunno how else to simplify my other suggestion, Rex.

I was kinda kidding.
Dec. 7, 2009 7:45 a.m. Tetzuoe Reader

happen to know the current draw on that thing? One of these might do the trick: ebay

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