charging 7AH sealed lead acid battery

Submitted: Saturday, Apr 26, 2014 at 15:45
ThreadID: 107473 Views:5825 Replies:8 FollowUps:7
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I am using a 7AH rechargable sealed lead acid battery to run a depth sounder. My battery charger is only suitable down to 14AH batteries so I am investigating the best way to recharge this battery. I have a solar panel for recharging car batteries, the type that sits the solar panel on the dash and plugs into cig lighter. This outputs about 50 mA. Would this be suitable. I have a battery booster pack at home that is charged with a 12v 1AH charger and wonder if that would also be satisfactory as suspect it will also be fitted with a sealed lead acid battery. I would appreciate advice from someone already doing this as didn't really want to buy another battery charger.

Thanks Kc
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Reply By: HKB Electronics - Saturday, Apr 26, 2014 at 16:24

Saturday, Apr 26, 2014 at 16:24
The solar panel would do at a pinch just check that it has a regulated voltage output of around 13.8V if you intend to leave on float charge.

Same for the thumper, it should be ok and I would expect it to also have a regulated output of around 13.8V



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AnswerID: 531266

Reply By: Member - Scrubby (VIC) - Saturday, Apr 26, 2014 at 19:36

Saturday, Apr 26, 2014 at 19:36
G`day Kc,

If you can find one of the small black chargers that are used with some electronic stuff that have the three pins to fit a normal 240v power outlet as part of it they are just the ideal thing, there are often boxes of that stuff at Sunday markets,garage sales ,etc.
Read the label to make sure it says "Battery Charger" and not "........ Adapter", it also should say what the voltage and output amps are.
For a 7ah battery 600ma or 800ma is good.
You may be able to buy one at the likes of Jaycar, Dick Smiffs, etc. not very exy I wouldn`t think.

I use one of these to charge the same battery as yours for my sounder.

AnswerID: 531285

Follow Up By: Member - Scrubby (VIC) - Saturday, Apr 26, 2014 at 19:40

Saturday, Apr 26, 2014 at 19:40
....forgot to add, cut the end off the lead wire and fit the correct size female spade fitting that fits the male terminals on the battery.

FollowupID: 814278

Reply By: Member - Phil G (SA) - Saturday, Apr 26, 2014 at 20:44

Saturday, Apr 26, 2014 at 20:44
Give the little solar panel a miss - 50mA is not enough (won't be enough hours in the day!)
It is quite likely that your 12V 1A charger is suitable. Measure the voltage while charging the battery and if its around 13.8V, then you're OK.
Also for $20 you can buy a SLA battery charger from Jaycar.

You can also simply charge it in your car. Jaycar sell a fused male/male cig socket for this purpose. On previous vehicles, I charged the 7Ah batteries via a relay that is powered whenever IGN is on.
AnswerID: 531290

Follow Up By: Member - Phil G (SA) - Sunday, Apr 27, 2014 at 08:48

Sunday, Apr 27, 2014 at 08:48
I'll withdraw the suggestion to directly hook up to 12V in the car if Bantam got one to take 12Amps. I've never seen more than 3 amps go into mine on a bench power supply, but mine have usually been 2nd hand SLA batteries.
I saw one melt once when it fell over in the back of someone's car and shorted the terminals, so make sure the terminals are not exposed.
FollowupID: 814311

Follow Up By: The Bantam - Sunday, Apr 27, 2014 at 10:49

Sunday, Apr 27, 2014 at 10:49
Yeh a flogged old battery will have a much higher internal the same session I tried an older battery.....while it got warm and the charge current was way above spec..nowhere near as dramatic as a near new battery.

FollowupID: 814326

Reply By: The Bantam - Saturday, Apr 26, 2014 at 22:28

Saturday, Apr 26, 2014 at 22:28
There are a couple of issues with charging 7 AH sealed lead acids.

They are a small battery and in general have a limited maximum initial charge rate.

Y want to charge em fast enough to be convieninet, but not so fast they get hot and blow up.....serioulsy...... I've seen em with the top split of em and blown up like a balloon, when the vent valves have not worked, in installations that where not properly current limited.

Best maximum charge rate is between 1 and 1.5 amps.

If you have 240V available, one of the cheap common sealed lead acid chargers from jaycar, altronics or similar will work well.

I run several of these.

At this rate they will charge a near flat battery in about 10 to 12 hours....a half flat battery in about 6 to 8.

they mostly have a light indicator that shows charge ready.

Charging a small sealed lead acid from a vehicle is a little more complicated.

I have done some research and some actual experiments.
If you connect such a battery direct to the charging system of your car you will very likely over heat it.

I hooked up a fairly healthy near new 7 AH sla direct the the battery of a vehicle with made up cables and with the alternator running at 13.8 vots, it got rather hot rather quckly and arround 12 amps flowed.....disconnected rather smartly.

SO if you want to charge in your car, you have to introduce some sort of current limiting.

People have used light bulbs, and they are moderately effective...others have used resistors.

I have tested both......A light bulbs is quick easy and no maths required...use a 12 volt 21 watt light bulb in series with the positive lead and all will be well.....but the battery wont charge real can move up to bigger light bulbs or more light bulbs....but ..yeh there are better ways.

A good quality high power resistor is far more predictable and far more consistent than a light bulb...but there is a trade off of current limiting V charge speed.....sorry cant remember resistor values but ther where under 10 ohms

Its not easy to calculate, you have to establish the value by experimentation.

People have raised the method of using three terminal voltage regulators....they are great if you have enough voltage to spare... they will regulate to the correct voltage AND do the current limiting for you.....BUT you do need a good 18 volts or more before the regulator to achieve a reliable 13.8 volt output to the battery.
At some point ...about 2 to 3 volts above the charge voltage the regulator will,shut down.

So the 3 termainal voltage regs are no use in the car..

Wat has been proposed is some sort of switchmode inverter to achive sufficient voltage off the 12 volt supply and the regulate down to the 13.8 volts with a 3 terminal regulator...yep that will work....the quickest way to achieve this is to buy a dc to dc converter lap top computer supply and regulate down to 13.8 volts with a 3 termianl reg.

All this can be had from Jaycar or altronics.

there is an easier way.

There re pre built ready to go boards that will do this available from ebay.....what you want is a DC to DC converter board that is current limited at arround 1.5 amps or has an adjustable current limit.

I have not got arround to trying any of these ...but on paper that would be the best solution with the fastest practical charge rate.

Hope this all hepls

AnswerID: 531295

Reply By: bks - Saturday, Apr 26, 2014 at 22:50

Saturday, Apr 26, 2014 at 22:50
Jay car have plenty of charges for them at about $20 to $30 dollars
AnswerID: 531297

Reply By: kcandco - Sunday, Apr 27, 2014 at 12:12

Sunday, Apr 27, 2014 at 12:12
Thanks everyone for your input... much appreciated. there is a lot of detail here and i will need to re read a few times as I am interested in understanding how this works. I have dug through my bin of chargers and adapters and found a ryobi 12v 400mA charger. I hooked it up to the battery and it looks like it will fit the purpose well. It is on charge at the moment and it will be interesting to see what voltage it takes the battery up to. I dont need a quick charge so 400mA will be fine. While going through the bin i was surprised to find battery chargers, adapters and power supply printed on the various items although to my untrained eye they all seemed much the same. The power supply for the battery booster i already have does not say what it is, just output 12v 500mA. The 240v charger that came with my uniden uhf handheld radio says it is an adapter so this adds to the confusion. I expected it to say charger. I also googled the voltage regulaters and found that quite interesting. It leads me to this question. What is the difference between a power supply, adapter and battery charger? Would they all be regulated to 13.8v if they are intended for 12v use?

Thanks again

AnswerID: 531320

Follow Up By: Zippo - Sunday, Apr 27, 2014 at 14:23

Sunday, Apr 27, 2014 at 14:23
The important thing to check on ANY of those before connecting them to a battery is what their open circuit voltage is. Many of these devices are simply unregulated transformer/rectifier combinations, with OC voltages as high as 17 or 18 volts. Not suitable as after any bulk charge the battery voltage will rise too high - unless this is being monitored with a view to disconnection.

Having satisfied that test, the remaining unknown is whether they take kindly to their input being turned off while the battery is connected. Most of the simple unregulated types do (ironic, as they are the ones unsuitable to charging ...) but many regulated devices don't. I have blown up more than one power supply when the mains supply has suddenly failed when charging a battery. In general a regulated power supply isn't guaranteed to be suitable in this role, and there is no easy (non-destructive) test that I can think of. If a circuit diagram of the electronics is available it can shed light on it to the trained person, but the layman is at a disadvantage here.
FollowupID: 814338

Follow Up By: kcandco - Sunday, Apr 27, 2014 at 18:05

Sunday, Apr 27, 2014 at 18:05
Thanks Zippo... very interesting. I just checked ryobi charger with no load and it read 13.21V. Tried the charger that plugs into the battery booster jumpstarter and it read 19V. Now I am wondering if this is the original charger for this pack or has been swapped at some stage. The jumpstarter is 15 years old and still going strong. Incidentally I have checked the small solar charger before and am pretty sure it was reading something like 18V with no battery connected. I will check this when the sun is on it again.

cheers Kc
FollowupID: 814354

Reply By: KenInPerth - Tuesday, Apr 29, 2014 at 19:58

Tuesday, Apr 29, 2014 at 19:58

Just to throw something else in here that is a little out of the square, it sounds like a standard 12V 7AH battery they use in most small UPS's.

Some of these UPS's are small enough to not be a pain in the butt to use as a simple battery charger - while they are not "smart" they are designed generally to re-charge and float these types of batteries.

If you had access to one of these it may be a good simple ready made option.

I have heaps of them if you are in WA, but if not you may find any larger IT company (not retail stores) that service businesses would have old ones they may have recovered from businesses.

They usually "don't work" simply because the battery in them is stuffed and not because the UPS itself is faulty. As long as you hook up a reasonably healthy battery the UPS will normally start and recharge the battery. A little adaptation to bring the internal battery leads external to the case and you have a pretty simple 12V SLA charger for 7AH batteries. Note however they probably will not have short circuit protection on the 12V leads as a normal charger does.
AnswerID: 531499

Follow Up By: kcandco - Tuesday, Apr 29, 2014 at 21:12

Tuesday, Apr 29, 2014 at 21:12
Interesting thought as I have a ups sitting under my office table. Cheers
FollowupID: 814517

Follow Up By: Zippo - Wednesday, Apr 30, 2014 at 11:47

Wednesday, Apr 30, 2014 at 11:47
Ken, it's interesting to look at the charge regime used in UPS units, and small ones in particular. Like you I have had quite a number cross my bench, abandoned because the SLA was shot.

Manufacturers of the "office" UPS aim for a v.quick recharge with a view to maximise the availability. They also tend to have too high a float voltage, killing batteries prematurely as no doubt you have noticed.

Before using one as a charger I would check the above (recharge rate and float voltage).

I have also used a small (150VA) unit as an inverter to power a 240V soldering iron in an emergency field job, but one needs to be acutely aware of the risk of electrocution so I wouldn't recommend that practice to others.
FollowupID: 814553

Reply By: garrycol - Wednesday, Apr 30, 2014 at 13:21

Wednesday, Apr 30, 2014 at 13:21
Surely any basic automotive batter charger you can buy from Repco or Supercheap will work fine. I have one that charges at 1.6 amps and another that is a max of 3 amps. They charge bike batteries Ok and car batteries that are not dead.

Something like this - click here
AnswerID: 531518

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