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What does this capacitor do?

ramsfan871

Sep 18, 2015
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Hoping for some help here. I'm good with mechanical stuff and not too bad with "electrical", but not so much "electronics".
Application is a bathtub lift chair for disabled people. Basically sits in the tub and goes up and down. Had to replace the rechargeable battery. The old battery has what I think is a capacitor on the positive post. The closest thing I could find (from a battery store) is a battery that matches voltage and amps but does not have the capacitor(or what I think is a capacitor).
What is the "capacitor's" purpose in this application and will it hurt anything if I install the new battery without it?
First picture is the old battery, second is the new. Third and forth is the case the battery is in.
 

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Harald Kapp

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Unfortunately your pictures are not too clear. Can you post a close up image of the supposed capacitor? Preferably such that we can se the inscription.

Although whether it is a capacitor or some other component: it has two leads. Where are these connected? Connecting both to the same contact doesn't make sense. I'd expect themto be connected between the two battery poles or between one pole and case. Or ???
 

duke37

Jan 9, 2011
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I think the device is in series with the battery, if so, it could be a resettable fuse.
In any case, it could be retained and only the battery replaced. It just plugs in.
 

Alec_t

Jul 7, 2015
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What markings are there on the battery? If it's a lithium battery the device may have something to do with protecting the battery.
 

Old Steve

Jul 23, 2015
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What markings are there on the battery? If it's a lithium battery the device may have something to do with protecting the battery.
Yep, that would be a good reason to put a polyswitch in series with the battery lead.
 

Harald Kapp

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Off topic: You're suddenly loking so much younger, Steve :confused:
 

ramsfan871

Sep 18, 2015
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It looks to me like this might be an add-on by the lift chair manufacturer. Since I don't know if this is a capacitor or what I'll just call it "The Device" from here on out.
I've added some better pictures. The second picture shows one lead from the device is soldered to the battery post itself. The other lead of the device is soldered to a flat washer looking thing. The third picture shows underneath the flat washer thing is a plastic washer thing separating the battery post from the flat washer. The rest of the pictures show that a spring just slides onto the negative post of the battery and another slides/sits on top of the washer thing over the positive post. Then a couple metal caps over the springs and the battery cover over that encasing the battery.
Could this be a safety feature since the application is a bath chair that sits inside the tub and there's a possibility of the battery being dropped into water?
 

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Old Steve

Jul 23, 2015
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It looks to me like this might be an add-on by the lift chair manufacturer. Since I don't know if this is a capacitor or what I'll just call it "The Device" from here on out.
Most definitely not a capacitor. A capacitor in series in a DC circuit would stop it operating. The cap would only conduct for a tiny fraction of a second, then stop.
"The Device" is definitely a polyswitch / polyfuse, and also definitely an add-on by the manufacturer. It wouldn't come on the battery when you buy one, except from the chair supplier.
It's just a fuse. It can be unsoldered from the old battery and fitted to the new one. Then, since you say the new battery's voltage and Ah rating match, you're good-to-go.
 

ramsfan871

Sep 18, 2015
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Oh ok. I had thought of just going ahead and transferring it to the new battery but I wanted to understand it's purpose. I was(and still am) concerned if doing so will void the warranty of the new battery as money is tight right now. The positive post of the old battery was bent in the opposite direction to accommodate the polyswitch / polyfuse.
What is the polyfuse there to protect? The battery, the electrical/mechanical components of the chair, or protection from electrical shock if the battery is accidentally dropped in the water? Voiding the warranty is a concern but safety is top priority.
 

Old Steve

Jul 23, 2015
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Oh ok. I had thought of just going ahead and transferring it to the new battery but I wanted to understand it's purpose. I was(and still am) concerned if doing so will void the warranty of the new battery as money is tight right now. The positive post of the old battery was bent in the opposite direction to accommodate the polyswitch / polyfuse.
What you could possibly do is connect the polyfuse to the new battery's positive terminal using a crimp-type female spade connector, with a lead going back to the actual contact. Then you don't need to solder anything to the terminal. Note that when you buy a spade connector, these batteries don't have the standard 1/4", (6.3mm), spade terminals - they're usually a 5mm version. (Measure it to be sure.) Pity you're not nearby, I have heaps here.


What is the polyfuse there to protect? The battery, the electrical/mechanical components of the chair, or protection from electrical shock if the battery is accidentally dropped in the water? Voiding the warranty is a concern but safety is top priority.
The polyfuse would be there primarily to protect the battery against excessive current drain. Depending on the circuitry it connects to, it could also protect the circuit and battery in the case of connecting the battery backwards, in reverse-polarity.

With such a low voltage, it wouldn't be to protect against electric shock. You could safely touch both battery terminals with your fingers and feel nothing. It's only a 12V battery, SLA. (sealed lead-acid)
 

ramsfan871

Sep 18, 2015
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Ok. Based on what you are saying I'm thinking it's there to protect against excessive drain. When in it's in it's plastic housing it slides into a slot on the back of the chair. Only fits one way so it's impossible to connect it backwards.
I'll look into the crimp connector idea. I may have a couple of the smaller crimp connectors. If not I'll buy a few so I have extras.
Thanks so much for the information and advise Old Steve. You've been a big help.
 

Old Steve

Jul 23, 2015
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Ok. Based on what you are saying I'm thinking it's there to protect against excessive drain. When in it's in it's plastic housing it slides into a slot on the back of the chair. Only fits one way so it's impossible to connect it backwards.
I'll look into the crimp connector idea. I may have a couple of the smaller crimp connectors. If not I'll buy a few so I have extras.
Thanks so much for the information and advise Old Steve. You've been a big help.
No worries. Glad to help, especially on a project that's not simply for fun. (Though fun is good too. :D )
 

duke37

Jan 9, 2011
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Check that the fuse has a low resistance across it. I had a battery charger where the fuse did not reset.
Perhaps the battery is all right.
 

Old Steve

Jul 23, 2015
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Check that the fuse has a low resistance across it. I had a battery charger where the fuse did not reset.
Perhaps the battery is all right.
We can hopefully assume that the battery's voltage has been measured???
(If not, a measurement of this is in order.)

The point about the polyfuse possibly being cactus is a good one, too.
 
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