Hi everyone. I've got a 25 year old powered recliner. It has 2x 24V actuators and 1x 4-button switch, all of which plug into a control box. That control box also has 2x12V sealed acid cells wired in series plugged into it, i.e. a 24V battery input. Finally that control box also has a 24V external power supply plugged into it.

I'm having a problem where the recliner only works if both the battery is plugged in, and the power supply is plugged in. If the power supply is plugged in, but the battery is flat, then I can hear the switch working and the relays clicking, but the actuators don't move. If the battery is full but the power supply isn't plugged in, then the relays don't even click.

So I can't use the chair at all without the power supply plugged in, but the weird thing is that I also can't use the chair with the supply plugged in if the battery is dead. And the battery tends to die every 2 years, so I have to take it apart, track down some more new lead acid batteries, and then repeat the cycle where for two year it works again but only if it's still plugged into the mains too.

Any ideas on what could have gone wrong? I've replaced the power supply, the battery, and the fuse on the circuit board. Is there an obvious component on the circuit board which could have blown that would lead to this issue?

Actuator -

Battery -

Cell -

Circuit board -

Circuit board marked up with connections -

 

Welcome to AAC!

the recliner only works if both the battery is plugged in, and the power supply is plugged in.

It's possible that if the state of the batteries (through age or charge level) is such that they can't supply enough current for proper operation of the recliner, then the control circuitry inhibits operation to prevent some malfunction.

 

Or that it is intended to be plugged in, and battery backup is for emergency escape.

 

Thank you both!

I remember when I originally bought it, it definitely worked off just the battery, and every few months I'd need to plug it in to charge it up. But since then I've lost the charger and changed it for a generic 24V one, I've had to change the batteries so change them for similar 12V ones, and I've had to replace the fuse. So any of those could have potentially had an impact, or maybe something went wrong on the circuit, but it used to work off the battery when originally designed.

It's very possible that the control circuit has a safety break, for when it thinks that a battery is disconnected, since the system is expected to have a battery in place at all times. An alternative would be that it's set up to work with a trickle charger, and therefore it only ever sends current from the battery, not from the mains, which would explain why the relays click when it's plugged in but the actuators don't move if the battery is dead. But the bit that doesn't explain is why, even with brand new fresh batteries, it no longer works without the mains charger plugged in. Is there any way of being able to decipher the structure of the main 24V flow from the circuit board - where the current from the power supply goes, does it go to charge the battery or does it go straight through to the actuators via the relays, etc.. My electronics knowledge is very rusty, and it might be too complex a task, but if there's something easy that I'm missing I'd appreciate some advice.

 

But since then I've lost the charger and changed it for a generic 24V one

I think the answer is right here. The new charger is not working correctly.

 

Is the generic charger actually specified as a charger for SLA batteries, or is it merely a 24V power supply? (It takes more than 24V to charge a nominally 24V battery).

 

Do you have a digital multimeter.
If so, what is the open-circuit voltage of the batteries after they have rested for 12 hours?
It should be near 12.9V. for a fully charged, sealed lead-acid battery.

 

Ok so this is probably where I went wrong in that case.

Having looked it up, this is the official charger for that control box. Whereas I'm using this generic power supply set at 24V. My understanding, clearly wrong, is that as long as the power supply is the right voltage, and as long as has a high enough wattage to supply more current than needed, it should be fine. I'm guessing that's not the case for a battery charger, right?

I do indeed have a digital multimeter, and when installing two brand new batteries, they do hover at the 13V level. My current old ones are down to 7V each, and that's the level at which the actuators stop moving and I need to swap them out. They're about two years old, and I've had a charger plugged in the whole time.

Do you think by plugging in the generic charger I've damaged the control circuitry?

 

Unlikely the 24V supply will have damaged the control circuitry, but if your battery voltage has dropped to 7V the battery may well have been damaged.
I see the spec for the official charger is "Charging voltage: 28 V + 0.3 V/ -0.6 V", with a max charging current of 0.5A.

 

Nice clear pictures.
Good luck..............

 

when installing two brand new batteries, they do hover at the 13V level.

How can they be 13V if you were charging the two with a 24V source?

 

How can they be 13V if you were charging the two with a 24V source?

To clarify, each separate battery shows about 13V, and they're wired in series in the actual circuit so voltage is doubled

 

Have a look here to see how SLA batteries should be charged.

 

To clarify, each separate battery shows about 13V, and they're wired in series in the actual circuit so voltage is doubled

Of course.
But you said you were charging them with 24V and 2 * 13V is 26V.
A battery voltage will never be larger than the charging voltage.
So you understand my confusion.

 

Of course.
But you said you were charging them with 24V and 2 * 13V is 26V.
A battery voltage will never be larger than the charging voltage.
So you understand my confusion.

He said new batteries were 13V each and now they are 7V each:

I do indeed have a digital multimeter, and when installing two brand new batteries, they do hover at the 13V level. My current old ones are down to 7V each

 

So your problem does seem to be not using a proper charger for the battery.
Getting a correct one should solve that.

 

Whereas I'm using this generic power supply set at 24V.

Assuming these batteries are in series, 24V isn't enough to charge them. Manufacturers of devices using series SLA batteries don't care about lifetime, so charging is a compromise. Once one of the batteries gets weak, both are going to die shortly.

When you replace them, always replace them in pairs with batteries of about the same condition. That's your best chance at getting matched batteries, but they're more likely to not be.

Series and parallel combinations are okay for matched batteries. Unfortunately, even when they start out that way, they don't stay matched for long and then the strong batteries will kill the weak ones. The same goes for the series cells in a 12V (or 6V) SLA. The battery is only as good as the weakest cell.

I do indeed have a digital multimeter, and when installing two brand new batteries, they do hover at the 13V level. My current old ones are down to 7V each

7V is a dead battery that is unlikely recoverable with a standard battery charger. Especially if it's in series with another battery. I've recovered a couple 12V/7Ah SLA batteries that had discharged to a few volts. It took months of using a battery maintainer to restore them. They now hold a charge for weeks, but I haven't tested to see how much capacity has actually been restored. I've had more that couldn't be restored than could...

 

when installing two brand new batteries, they do hover at the 13V level. My current old ones are down to 7V

It's not completely clear.
Do you have one old set of batteries and one new set?

 

Ok great, thank you. So yes, exactly, every couple of years I buy two new 12.9V batteries, and swap them in. I've just done this. The old ones are now showing about 7V each on the multimeter. The new ones come fully charged and are showing about 13V each.

It also now makes sense why the official charger is 28V even though the batteries are 24V. I was worried that I'd fry the circuitry, as I didn't know what voltage the original charger was (I only just managed to track it down online in the last couple of days), so when I lost my charger I bought a 24V one because the actuators were 24V. Clearly I should have got a 28V one, which would be enough to charge the batteries.

So this will solve why I keep having to change the batteries every 2 years, thank you - the original set easily lasted a decade, when I was using the original charger. I'm going to try and buy a replacement original charger, but it looks like it's not a particularly smart charger, it doesn't have any of the 3-stage charging functionality mentioned in the SLA battery charging guide linked above. So would you say that a generic 28V charger would do the trick?

But I still need to work out why the chair will only work with the charger plugged in, not just the batteries. Even with brand new batteries, showing full voltage at installation, the relays don't fire when I press the switch - which is definitely how the chair worked originally, and which makes sense since the official charger only does 500ma so can't possibly provide enough current two separate 5A actuators. So the way the circuit is supposed to work is the whole thing works off the batteries, and the charger just trickle-charges the batteries when necessary. But somehow it's ended up with the charger needing to be plugged in for the electronics to work, which doesn't even really make sense. Any ideas?

 

You keep saying you bought a 24V charger. You did not, you bought a 24V power supply. They are not the same thing. A charger has additional circuitry that depends on the battery chemistry.

Get a charger that is intended to charge 2 12V lead acid batteries in series and you will be fine.

If you go out now and buy a 28V power supply you will have even worse problems, so please don’t do that. Get a charger that is explicitly made for charging lead acid batteries.