12v gel battery charging questions

[the unmagnificent man]

Hi all. I have a 12v gel battery. it is a 7.2 ah (7200mh)

I have a multi function charger. I checks the type of battery so I can't accidently put the wrong type of charge in. ie nicad, Lipo, Nimh and lead acid (gel)

I can choose that it switches off after a chosen number of munutes, I can choose the amount of charge from 0.1amp to 5.0amps I can also choose the cut off cut off voltage, and the cut off mah (ie cut off at 2200mah)

To be honest its far to complicated for me as I don't know the best amount of charge to put in it.

So here are my questions

1. What is a good charge to put into a 7.2ah 12v gel battery? (from 0.1 to 5.0amps)

2. Can you charge these batteries before they are flat or is it best to wait until signs of low power?

3. Even though its a 12v do I set my charger to cut off at 12v or is it like a car 12v battery that charges up to about 13 or 14v?

4 given that I can cut off at 7200mah, what if the battery is approximately half charged do I still try for the 7200 or somewhere about 4000mah?

I won this charger so please dont ask why I didn't buy a simple one, I'm sure this charger would be the bees knees in the hands of the right person. I prefer mechanical problems as I can work those out.

Answers in laymans terms please as even some of the above is only there as I copied it from the battery. Thanks. UMM

[Steve Hargreaves - Moderator]

For that battery UMM I'd set a charge rate of about 1 to 1.5A & let it go......the charging algorithym won't let the battery voltage go above 14Volts & will gradually reduce the current to maintain the max volts at 14 until it gets to about 0.1A & then it will switch off.....

Make sure you set the charger to 12Volts or 6 cells...whichever option it gives you....

Charge it when you feel like it needs it....no need to flatten it first or worry that its nearly full.....the terminal voltage will tell the charger all it needs to know.....

HTH

[the unmagnificent man]

Thanks for that Steve, now I understand why on one of my batteries the charge was set to 0.5amps but after a while it dropped and then went back up again.

Lets hope now that its enough to power my power panel and turn my starter. UMM

[Simon Chambers]

Posted by the unmagnificent man on 21/11/2012 18:57:27:

Hi all. I have a 12v gel battery. it is a 7.2 ah (7200mh)

I have a multi function charger. I checks the type of battery so I can't accidently put the wrong type of charge in. ie nicad, Lipo, Nimh and lead acid (gel)

I can choose that it switches off after a chosen number of munutes, I can choose the amount of charge from 0.1amp to 5.0amps I can also choose the cut off cut off voltage, and the cut off mah (ie cut off at 2200mah)

To be honest its far to complicated for me as I don't know the best amount of charge to put in it.

So here are my questions

1. What is a good charge to put into a 7.2ah 12v gel battery? (from 0.1 to 5.0amps)

2. Can you charge these batteries before they are flat or is it best to wait until signs of low power?

3. Even though its a 12v do I set my charger to cut off at 12v or is it like a car 12v battery that charges up to about 13 or 14v?

4 given that I can cut off at 7200mah, what if the battery is approximately half charged do I still try for the 7200 or somewhere about 4000mah?

I won this charger so please dont ask why I didn't buy a simple one, I'm sure this charger would be the bees knees in the hands of the right person. I prefer mechanical problems as I can work those out.

Answers in laymans terms please as even some of the above is only there as I copied it from the battery. Thanks. UMM

 

1. Often it says on the battery. If it doesn't, then for Lead Acid batteries it's recommended to charge at C/3 (also termed as C3 or 0.3C - C = Capacity) for fast charge- i.e. 1/3 of the capacity, which is 2.4A charge rate for yours. This'll take 3-4 hours However for a long battery life, it's recommended to charge Lead Acid batteries at C/10 (also termed as C10 or 0.1C) for slow/float charge - i.e. 1/10 of capacity, so 720mAh (or 700mAh). This will take 12-16 hours.

2. Always keep a Lead Acid battery fully charged when not using it for a long battery life. These do not like being left discharged at all. Don't flatten it either, as it doesn't do it any good. Best to not discharge more than 50% of capacity for long life.

3. It'll charge up to around 14 to 15v like a car battery. Don't need to worry about the exact voltage, however make sure you select the correct battery nominal voltage on your charger - which is probably 12v.

4. Don't worry about the cut off, infact for charging the Lead Acid keep it well above (i.e. 10000mAh) - otherwise it may say it's finished before it really it is.

Si.

Edited By Simon Chambers on 21/11/2012 20:37:25

[the unmagnificent man]

Thanks to you both for taking the time to explain. I learned a bit and now understand a little more. Thanks. UMM

[John Cole]

If it's a sealed gel lead-acid battery (SLA) then I think Simon's advice is incorrect. You MUST use a charger that cuts off at the correct voltage. Unsealed car batteries can safely be charged above their full-charge voltage as the electrolyte then breaks down releasing oxygen and hydrogen (which is why car battery chargers say you must use them in a well-ventilated space as this mixture is very explosive). If you do this with a SEALED battery ...

Special chargers for sealed gel batteres cost just a few pounds and cut off at a safe voltage.

On-line checks suggest a fully charged SLA battery will read 13.65 volts and this will drop when taken off charge to 13.2 volts after several hours.  This link suggests fast-charge to a target of 14.45 volts with a drop to constant-voltage charge at 13.65 volts. The charger referred to is for a very small SLA: 1.2 AH. so the peak charge current is just 300 mA.  Yours is 6 times bigger and that suggests a peak charge rate of 1.8 A / 1800 mA.

Edited By John Cole on 23/11/2012 12:51:12

[Simon Chambers]

Posted by John Cole on 23/11/2012 12:41:24:

If it's a sealed gel lead-acid battery (SLA) then I think Simon's advice is incorrect. You MUST use a charger that cuts off at the correct voltage. Unsealed car batteries can safely be charged above their full-charge voltage as the electrolyte then breaks down releasing oxygen and hydrogen (which is why car battery chargers say you must use them in a well-ventilated space as this mixture is very explosive). If you do this with a SEALED battery ...

Special chargers for sealed gel batteres cost just a few pounds and cut off at a safe voltage.

On-line checks suggest a fully charged SLA battery will read 13.65 volts and this will drop when taken off charge to 13.2 volts after several hours. This link suggests fast-charge to a target of 14.45 volts with a drop to constant-voltage charge at 13.65 volts. The charger referred to is for a very small SLA: 1.2 AH. so the peak charge current is just 300 mA. Yours is 6 times bigger and that suggests a peak charge rate of 1.8 A / 1800 mA.

Edited By John Cole on 23/11/2012 12:51:12

Most chargers for Lead Acid batteries charge using Constant Current and then to Constant Voltage, once it hits the maximum charge voltage - this is what Car Alternators do (hence the regulator packs on the back of them). As you've quite rightly pointed out, different formats of Lead Acid batteries have different voltage levels for the Constant Voltage section. The real cheap (and crap) car battery chargers that only charge as a trickle charger tend to just have a half wave rectifier straight off the transformers output. As this isn't regulated, this can put a voltage above the maximum float charge voltage. So like you said, on a flooded battery (as your typical car battery) this can be tolerated as it produces hydrogen+oxygen. However it's not recommended for long periods of charging, but it can sustain it if the charge rate is low enough.

Any other type of charger and especially those with a fast charge facility, is likely to have a proper Constant Current and Constant Voltage modes. Going back to UMM's charger, as he stated that it has a interface to specify battery type, current, etc it's almost certainly going to charge the battery correctly (CC/CV). However I didn't want him to think that he had to set the correct end voltage before charging rather than setting the nominal voltage. As setting the charger to charge as a 7S pack (14v - which would be roughly the correct CV mode voltage for a 12v/6S pack) will severely overcharge it. So this is why I said not to worry about the exact charge end voltage on the screen (when operating), just worry about getting the correct nominal voltage selected (e.g. 12v) - before charging.

Lead Acid batteries can be safely overcharged - along as the current is limited to the correct level (depending on the pack manufacturer, it can be C/3, C/4 or even C/5). When over charging the hydrogen and oxygen that is generated by the electrolytic action will get recombined. However there is a limit at the rate which it can do this, hence the maximum charge currents. If it wasn't possible to overcharge a battery, then Lead Acid batteries would have a balance connector (like LiPo's do), more intelligent chargers and for most applications would need a Battery Management System.

Si.

[John Cole]

Posted by Simon Chambers on 21/11/2012 20:36:28:

3. It'll charge up to around 14 to 15v like a car battery. Don't need to worry about the exact voltage, however make sure you select the correct battery nominal voltage on your charger - which is probably 12v.

4. Don't worry about the cut off, infact for charging the Lead Acid keep it well above (i.e. 10000mAh) - otherwise it may say it's finished before it really it is.

Don't worry about the cut-off voltage or current? I disagree.

And so does the manufacturer of the SLA charger I linked to.

[Simon Chambers]

Posted by John Cole on 23/11/2012 20:00:31:

Posted by Simon Chambers on 21/11/2012 20:36:28:

3. It'll charge up to around 14 to 15v like a car battery. Don't need to worry about the exact voltage, however make sure you select the correct battery nominal voltage on your charger - which is probably 12v.

4. Don't worry about the cut off, infact for charging the Lead Acid keep it well above (i.e. 10000mAh) - otherwise it may say it's finished before it really it is.

Don't worry about the cut-off voltage or current? I disagree.

And so does the manufacturer of the SLA charger I linked to.

Depends who has to worry about it. UMM doesn't, as the charger does it for him - hence not needing to worry...

He doesn't need to worry about the maximum charge voltage (when it hits CV mode), as that is handled by the charger.

Also there isn't a need to worry about the cut-off capacity (hence the numbers given in mAh), as the charger will go into CV mode when it hits the the Constant Voltage. The cut-off capacity is usually used for NiCd/NiMh mode which only charges in CC until it hits the delta-peak - but it can miss the delta-peak and severly overcharge a pack and killing it. In fact, in Lead-Acid charge mode, it may not even use the cut-off capacity setting.

Note, a charger won't "cut-off" on voltage or current when charging a Lead-Acid. It will go from a current limited mode (i.e. Constant Current) to voltage limited mode (i.e. Constant Voltage). Depending on the charger, it may drop the voltage in CV mode once it deems the pack is charged (usually when the current draw on CV mode has dropped below a defined level), but it certainly won't cut off the voltage or current once it hits it - otherwise the pack will never be charged!!

Si.

[Peter Beeney]

From a substantial amount of swanning around SLA’s cells, I’m inclined to think that in practise they are not as tricky as they first appear. The design and construction is such that basically they cannot be overcharged. If they do generate any internal pressure there is a relief valve to release it. This is set at around 4 psi, I believe. And is also why they are known as valve-regulated cells, which figures. But this is considered a very low risk, the larger industrial types are placed together with the electronic equipment they supply; in vast numbers. If there were any chance of any acid fumes escaping this would not be practicable. We did once try and cause this valve pressure release to occur with some old cells, but they were quite big, in capacity, and we only had a relatively small voltage source at that point, so it was an exceeding wet squib effect. Nothing happened!

There are some small one’s, Cyclon cells, cylindrical in shape and from about 2.5Ah up to 25Ah. I’ve been involved with these considerably, too, and they are remarkable, with a very low internal impedance they can deliver the short but high current pulse goodies! The 2.5Ah titchy weight can dispatch 65 amps and the 25Ah 250A. Also they seem to accept any form of charging, over a long period of time, as we proved for a number of years. Many SLA’s were in a situation where the voltage was above the recommended 2.3 volt/cell, for a long time, years, without coming to any harm. There is another type, Bolder Technology, which I believe are American, and I’ve never been able to source these, unfortunately. Because they are phenomenal! A 1.2Ah model weighs 77 grams and can be fully discharged at 30C and then be recharged in six minutes. In power tools they are said to out-perform nicads.

I’ve related this story before, but I lobbed a SLA on my van, with a 2 litre diesel engine. It was a Gates battery, about a quarter of the size of the standard battery, which meant I had to pack the compartment out with chunks of polystyrene foam. Sited under the bonnet, next to the engine. 100Ah rating, and made to an aviation spec. This ran for five years, and never once showed any signs of flagging at all. It was still on the vehicle when I sold it. I think that was a good, practical field test. And it passed!

Talking of cars, a car alternator is virtually an unrestricted constant voltage source. It has to be, on say a modern luxury limousine under the right circumstances the current demand can be considerable. The alternator has to supply all of this and more. I believe the alternator on my basic 1.6 litre diesel is rated at 180 amps, but I’d need to confirm that. The generally recognised voltage level for the alternator output is 14.2 volts, but again I believe that some vehicles do automatically raise this level slightly in the winter to ensure the battery is constantly well maintained.

The float voltage for lead acid cells is 2.3 volts/cell, but if the temperature is consistently above 25 C this is reduced to 2.27 - 2.29 volts/cell. A large cell can start to warm up in the middle, and the heat might not disperse quickly enough. Occasionally the boffins would come along and try out another of their hair-brained schemes, and the perceived wisdom was always that the voltage window tolerance of SLA’s was wider than that of flooded lead acid, you could charge them to a slightly higher voltage, and discharge slightly lower.

Just to be different, I consider a lead acid charger is a constant voltage source with variable current limiting. The voltage output might be at say 2.3 volts/cell, but we place a limiting device in circuit, which we can choose, by selecting say 2 amps. This limits the current flow to 2 amps until the difference in voltage between the charger and the battery is very small, the current then reduces and eventually becomes nothing.

My model battery charger will charge a 12 volt lead battery to 13.8 volts, which is fine, but it’s not completely charged. We have an electric fence at the patch, I charge the battery, normally to this voltage, but occasionally, once a year maybe, I raise the voltage to about 16 volts, at a low current, for a while. This is an ‘equalising’ charge, and it ensures all the cells are fully charged. But if one cell is faulty, it make no difference to this. The faulty cell will run out steam before the others, less capacity, and so on the next charge it will be fully charged before the others, and thus get a degree of overcharging, and continue to compound the problem. I don’t know this, but I would think it would be unlikely that you can adjust the end-point voltage on a model battery charger for lead acid. That would require some extra circuitry, cost?, surely, and also lead to complications.

I’ve not seen this, but I believe that large batteries do have quite a sophisticated management system. It started in a small way before I left work. Each cell in the battery would have a separate connection, exactly like a LiPo balance lead, only with many more outlets, this would be connected to a cascade box, so that each cell could be interrogated separately as required, and this in turn connected to a microprocessor so that it could all be done from a remote point. And nowadays I’m sure by a computer.

I use a Cyclon 2v 8Ah as a glow driver, and it certainly does not get any care at all, but when I do eventually get to charge it, it charges just fine, which is the first sign all is well. You can tell the health of a battery by the way it charges; and this one is a few years old now.

PB

[kc]

I decided to keep my flight box battery in use even though I mostly fly electric - I just use it to recharge the Lipo at the field. Good job I kept it charged because last weekend on a beautiful flying day my car wouldnt start due to having left the interior light on accidently. 40 hours and the car battery was flat!

Decided to see if my little 7 amp hour flight box battery would start the car. It did! Thats a result I didnt expect. Worth remebering.

[Peter Beeney]

I’ve seen that done too, kc, once before, on a diesel car, they usually require a fair old dollop of amps for the heater plug supply first, as well, for a few seconds; and so like you, I would not have been at all surprised had it not have worked! Quite amazing! There is a report that those little Bolder cells I mentioned, six in series, started a car ‘numerous times’, and was then recharged in ten minutes, but I don’t have any other specific details. That sounds even more impressive.

It’s the cold weather that sorts out batteries. Another diminishing returns thing. The engine becoming increasingly more difficult to rotate but with the battery capacity becoming an ever less amount to rotate it with! Which is why the battery is now often rated in cold cranking amps, the current it can deliver for 30 seconds at minus 18 degrees C, down to 7.2 volts. It’s very important to always keep lead acid cells fully charged anyway, but particularly so in the winter.

Re. the OP, if I had one of the 12V, 7Ah batteries as a field box supply and I had an accurate 13.80V supply I’d just leave that permanently connected on charge when it wasn’t in use. If used in the charge / discharge mode, then a 14 - 14.5 volt charge at least might be a better bet, but perhaps not always so easy.

As I also posted in a previous thread a while back, what I’d also do is use a dedicated charge connection to be able to keep the positive charge wire separate in the flight box and then just pop an inline fuse holder in this wire close to the battery terminal. The continental type fuse holder is fine, very low cost from a Motorist Centre. Complete with a 10 or 16 amp fuse, this is to protect the wiring in the event of a short in the charger. If this happens with no fuse protection the wires will be alight in a few seconds and the flight box is perhaps not the most suitable place for this to happen….. Particularly if it’s indoors…….

Not a scare story, please, but it can happen.

PB

[the unmagnificent man]

Wow,I feel like I just read a book (not a common thing for me). Thanks for all the pointers, I will look into the fuse its a good idea.

Since posting I noticed that when "12v Gel cell" (pb on my charger screen) is chosen that it will not allow me to increase the charge above 12volts, but having said that the charger then goes on to charge up to around 14. something volts.

I charge at 0.3amps (300mah?) and noticed that the charge rate dropped to 0.2 and then 0.1 before displaying "FULL"

Im glad its clever because I'm not when it comes to electrickery.

Thanks to all who posted.

[Peter Beeney]

UMM, - 14 something volts sounds about right. Charge it up when you are not using it, and keep it topped up. It’s called a 12 volt battery, but it has to get to 14 something volts to be charged. Put it on charge any time, you can’t overcharge it, it’s just a constant voltage charger, and that’s exactly what should happen.

Home and dry, then…

PB