Battery & charger question

[Tim+]

A friend who is having a hip op has been lent a battery powered bath lift. (Like this:

)

Unfortunately they've lost/mislaid the charger. I had a look at it and it's a sealed 12 lead battery (6Ah I think). When turned on, an LED flashes green, turns to red and then goes out. Nothing then happens so I assumed that the battery was dead.

Replacement chargers are pretty pricey so I got to wondering if I could charge it with my "smart" Lidl/Aldi car battery charger.

Before doing anything, the voltage measured 12.5. I connected my smart charger and after deliberating for a a few minutes it allowed me to turn it on but then said that the battery was fully charged. Given the inoperativeness of the bath lift I suspect that this is wrong.

I then dug out an old "dumb" charger. Connected it and monitored the battery to make sure it wasn't getting over-warm. After a couple of hours. The voltage had risen to 13.5 and seems to be holding there. Unfortunately I only have the battery, not the lift but would it be reasonable to assume that it's now charged? I curious as to why the smart charger was happy with just 12.5V. Would this normally pass as a "charged" battery or is it just down to the mismatch between its intended purpose (car/motorcycle battery charging) and this small battery?

Next question, is this a "safe enough" method of charging the battery for relatively short term use? The dumb charger is an old halfords 6amp charger with a high/low selector switch. I only tried it on "low" and the ammeter barely flickered (although the battery did get "tepidish" after a couple of hours.

The friend isn't at all technical and I don't want to lend them an unsafe arrangement.

Tim

[Capitol]

May be wrong, but I don't think it's the battery. A Lidl charger will work OK IMO. The Lidl unit drops back to trickle charge when the battery is up to volts. The battery should work down to about 10.8V under normal circumstances, unless it's clapped. Put a headlamp bulb on it as a load and see what happens.

[Tim+]

I suspect the electronics in the bath lift are very "fail safe". It's designed to never leave you stranded in a bath without sufficient power to lift you out again so I wonder if it's very picky about voltage? I tried a headlight bulb and under load the voltage drops to 12.5 and then rises back up after disconnection.

As for the Lidl unit, I've had my doubts about it for a while. It seems to have a mind of it's own.

Tim

[Harry Bloomfield]

Tim+ brought next idea :

The relatively tiny battery, would rapidly be brought to full charge by a 6amp uncontrolled charger. The 6v/12v lidl charger, if the battery is too low, will assume its the 6v and show it as fully charged.

Just because it seems to hold 13.5v, likely means the battery is showing just a surface charge. A fully charged 12v battery, left off charge for several hours, should show 12.7 to 12.9v.

The only way to prove a battery is good, is by it holding a charge for a good amount of time and supporting a load for a sensible time.

[Harry Bloomfield]

Tim+ was thinking very hard :

That sounds promising.

.. But how does the lift behave when connected to the battery?

[Capitol]

I also found the Lidl units to have a mind of their own. They can't cope with calcium batteries IME

[Tim+]

If I had the lift, life would be simple. ;-) Currently it's 40 miles away. I plan to reunite them soon but I want to know if it works okay, would it be okay for my friend to use my dumb charger to charge it with?

Tim

[Harry Bloomfield]

It happens that Tim+ formulated :

No, that would be a bad idea. He would need the lidl unit or a working version if it is indeed faulty.

[Dave Plowman (News)]

A lead acid reading 12.5v (after being left to settle) is pretty well fully charged.

My guess is the battery is knackered. It's fairly common to get a 'normal' voltage reading where one or more cells have gone high impedance and won't deliver any appreciable current. And, of course, a DVM takes near zero current.

Check the battery voltage under load - a car headlamp bulb or MR16 etc. A few amps at least. If the voltage drops appreciably, you've got the answer.

[Tim+]

Care to be more specific as to why? Let's pretend that they could understand instructions like "only charge on the low setting" and "never leave the house whilst it's on charge" and "feel the battery frequently and make sure that it never nets hot". Would following that sort of regime make it "safe enough"?

I could go out and buy a new smart charger but as this is probably only going to be used for a couple of weeks I'm not in a great hurry to buy a new charger (that might not work with this small battery).

Tim

[Tim+]

See other post. Only drops to 12.5V with car headlamp bulb.

Tim

[Dave Plowman (News)]

I had a Halfords battery marked 'calcium' and it worked ok with that.

[meow2222]

safe as in wont hurt anyone, yes. safe as in wont trash the battery, no. If you monitor the charging constantly and disconnect when V reaches 13.6-13.8v it'll be fine. Let V rise higher and the SLA will have a short life.

NT

[Harry Bloomfield]

[e-mail] was thinking very hard :

Well, that is the main reason, but SLA's also have the potential to explode in your face, if charged at too high a rate and via an uncontrolled charger. They can build up heat and pressure.

[Peter Parry]

SLA do not tolerate full discharge well. They also don't like charging on a non-voltage stabilised supply. The probability is the battery is damaged. You mentioned testing it on load - how long will it maintain that load?

Overcharging can lead to acidic electrolyte leaking out of the vent valve.

It is probable that the Lidl unit is OK but not happy with a damaged battery. I've a collection of SLA from such devices which the users have allowed to discharge and leave discharged for days or weeks. In the vast majority of cases the batteries are compromised. Some will charge from a Bench stabilised power supply but won't then hold that charge for any time. Some just eat electricity and store none. Most of the damaged batteries won't charge on an Aldi (same unit as the Lidl I believe) smart charger, all the good batteries will. It seems the smart charger is smarter than I thought.

[Tim+]

Okay. That's helpful. I haven't let it go above 13.5 so far. Be interesting to see how it functions when it's reunited with it bath seat.

Tim

[Tim+]

Dunno. I'm disinclined to discharge it if I have successfully charged it now. Unfortunately I'm not in a position to test it properly in the bath seat until next week.

According to the instructions, once charged, it should power the chair through many cycles so I might not need to recharge it again.

Tim

[meow2222]

No they don't. They have rubber valves that let out any overpressure. And a car charger isn't going to oevrcharge them enough to do anything dangerous. The risk is just that at too high a voltage they'll breakdown water and dry out.

NT

[Bill Wright]

That's not sensible.

Bill

[Dave Liquorice]

How long can it keep the headlamp bulb glowing brightly? A 6 AHr battery should manage 30 mins no trouble.