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Rechargeable battery question

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stuee

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Ok im trying to make a rechargable light for my boat.
I have a 12v/3w mr16 light which i beleive will consume 250ma.
this light will hopefully be on a sensor and be on and off for a few hours a day.
I my aim is to have a small solar panel at the top like on garden lights then i was going to get a 2.5ah battery but it was too bulky but now looking at the chinese AA batteries. These would be ideal as i can join them all together then slide into the pole the light will sit on.

My question is

the 12v sla battery is 2.2ah and the light on constant will last around 6hrs.

if i top > tail 10x AA batteries 1.2v / 3000ma would this then make it a 12v / 2v 3A battery and last longer with the light on?

also what would i need to hook a small solar panel upto this to keep the batteries charged in the day, the night light will only be used probably once every couple of weeks and be in the sun the rest.

thanks for any assistance.

cheers
 
It would take ball park 35mA to charge a 3000maH battery it 2 weeks, solar panels are usually optimistically rated so you'd need something a little larger than that.
You can just connect a diode in series with a panel, however in this application a shunt regulator would be a good idea to prevent overcharging.
To be honest your probably better buying something and cannibalising it to do your job.
 
another silly question but the voltage from the solar would have to be 12v so a 7v .85ma wouldnt do?
 
another silly question but the voltage from the solar would have to be 12v so a 7v .85ma wouldnt do?
The open-circuit voltage of a solar panel suitable to charge a 12V Lead Acid battery needs to be about 20V. The rating of such a panel will say something like 14V @ xxA. The Wattage of the panel would be the product of 14V and xxA.
 
Hello there,

You should also be aware that there will be some minimum value for the charge rate to charge a given battery. The approximation for lead acid charge time is something like t=1.4*AHr/A where A is the charge amps, but that does not follow all the way down to zero amps as there is a minimum for the charge amps that will charge a given battery. A 12AHr battery would probably need a min of about 300ma or something around that figure. Anything too much lower than that will not charge the battery anyway even if you leave it there for a hundred years. I dont have the exact figure on hand but you get the idea. 35ma will not be enough for example.
For a 2 AHr battery maybe you can get away with 100ma charge current and possibly 50ma, but it will take a very long time to charge that way.
 
Thanks for all your replies.
if i put 3 of these together to make 12v would this suit?
**broken link removed**
 
ok, might change my mind on the solar thing now as its starting to get a bit too big.
Perhaps i can have the battery inside the tube linked up. Is it possible to have a wire i can plug into the car cigarette lighter socket to charge the batteries when not in use and also if the batteries go flat i can jsut plug in the cig lighter to a waterproof socket in the boat and it will jsut run from the 12v boat source.?
Im not sure how to work it as i would like it not to charge the batteries from the boat battery when i plug into the boat but charge from 12v if i put on the car battery charger.
also would it work if I put 10 x 1.2v 3000ma batteries in line to make 12v but also add 4 more serial 3000ma 1.3v to up the ampage to last longer?

thanks
 
Hi,

As you must have read in the other posts here, if you want to charge a battery you have to have MORE voltage than the battery, and usually for a solar panel it has to be significant. As a rule of thumb, i would shoot for 2 times the voltage and 1/10 the ampere hour rating for the current. So for a 2AH 12v battery i would shoot for 24v panel at 200ma.

Just to be clear, to get these numbers we take the battery voltage and double it:
12 volts times 2 equals 24 volts,

and that gives us the open circuit panel voltage required.

Next, take the Ampere Hour rating of the battery and divide by 10:
2 Ampere Hours divided by 10 equals 0.2 which means 200 ma,

so we need a panel that can put out 200ma and has a 24v open circuit voltage rating.

Even so we have to hope that it charges well enough because there are other variables like the angle of the panel to the sun and cloud cover and stuff like that that is hard to control or predict. So the idea is to go with an overkill solution and that should at least work to some degree even though it might not be perfect.

Probably even a better number for the current would be to double that, to 400ma, and that would mean we get some charging even during periods of lower light levels.

This kind of approximation also helps us because the battery is often rated with a voltage number that indicates the discharging voltage, and the charging voltage can be higher. For a 12v lead acid for example, the charge voltage can be as high as 14.2 volts, so you can quickly see how a 12v panel would never be able to charge that kind of battery, we'd need at least 15 volts, and that is with a load current of 200ma not open circuit (open circuit is zero current in the panel where it is not charging the battery).

If you look around the web you can probably find load graphs of solar panels. As the load current increases, the voltage of the panel drops, so that is another reason why we have to shoot for a higher voltage panel than the battery. Twice the battery voltage is a good starting point, but a few volts lower than that could also work.

To get the most power out of the solar panel there is also a maximum power point on the panel curve which means at that voltage and current we get the maximum power we can ever get at the given sun light level. Sometimes this is controlled by changing the load with a control circuit to maintain the best load point. This gets a little complicated however because we have to add a control circuit and power converter.
 
A solar panel has its voltage rated with no load and its current is rated with no voltage (into a short circuit). That is stupid.
It should be rated with a typical load, and when it is directly facing the sun at noon. Then you wonder about how little current it produces when it does not face the sun and at different times of the day.
 
A solar panel has its voltage rated with no load and its current is rated with no voltage (into a short circuit). That is stupid.
It should be rated with a typical load, and when it is directly facing the sun at noon. Then you wonder about how little current it produces when it does not face the sun and at different times of the day.

Not quite. Solar Panels from reputable manufactures are sold by their Wattage. Here is a typical IV curve. If you buy a panel to charge a battery, then you need to get one that puts out its peak power at close to the battery voltage during charge, which would be ~14V for a 12V battery. Such a panel will have an open circuit voltage of >19V.
 
The tiny solar panel from e-bay is the same as on a cheap solar garden light and costs the same. Then the rechargeable battery, voltage step-up circuit, LED and housing on the garden light are free.
 
ok so i will give up on the solar panel as cant find a cheap 14v panel thats small enough to fit on the light so i will go the battery charger.
So here is my idea.
I have the 10x 1.2v batteries in the tube, at the top i have a waterproof 3way switch. battery / off / 12v.
The 12v deep cucle battery will have an anderson plug onto it so when i get home i jsut plug my charger into the boat and it charges my 2 big 12 batteries.

My question is if im out and the AA batts go flat then i flick the switch to 12v and isolate the aa's out of usage and plug the 12v cig lighter socket in so it uses my 12v system, if i leave it plugged in when i plug in the car battery charger to the anderson plug then switch to the aa battery will it be ok to charge the AAs or will it need a different type of charger for them.
I hope this made sense :)
 
I have never seen AA Ni-MH cells as high as 3000mAh. American Energizer and Duracell Ni-MH cells are 2300mAh.
They need a charger circuit that limits the current and detects when they are fully charged then it turns off. 10 cells will be about 14V to 15V when fully charged and the charger circuit might need at least 16V.
 
**broken link removed**

so even though a car charger is 12v and the paralell 10x aa 1.2v = 12v the car charger wont charge them?
 
Why do you look at cheap no-name-brand Chinese junk on e-bay?
I looked in Google at BTY Battery Manufacturer and found a review of the cheap batteries. The "3000mAh" batteries measured only 170mAh to 257mAh and might be less after they are used!
The number "3000" on the batteries is not the mAh, it is just a number picked at random.

A car battery is huge and charges with a high current. A Ni-MH AA cell is small and MUST have its charging current limited with a circuit or it might blow up!
Also, the charger circuit MUST detect a full charge then turn off.
One Ni-MH cell charges to 1.4V to 1.5V then ten in series make 14V to 15V, not 12V. They produce 12V when they are half-way discharged.

10 cells in series make 10V to 15V. 10 cells in parallel make 1.0V to 1.5V, not 12V.

Here is the review. The comments agree with it:

EDIT: the cheap Chinese cells weigh 20.6 grams. my American Energizer cells weigh 27.0 grams.
 
I have never seen AA Ni-MH cells as high as 3000mAh. American Energizer and Duracell Ni-MH cells are 2300mAh.
They need a charger circuit that limits the current and detects when they are fully charged then it turns off. 10 cells will be about 14V to 15V when fully charged and the charger circuit might need at least 16V.

**broken link removed**

Pretty close but you need to charge them with a bit of weight behind the charger. Charging such cells at C/100 will probably not charge them at all.
With that kind of capacity, I would use a balanced charger which is a little more complicated.

A few years ago I bought a solar cell to keep my car battery topped up while away for a six weeks wired directly across the battery and sat on the bonnet (it was parked in a secure compound). It had an open circuit voltage of 19v and a reported current of 50mA output.

Waste of time. We had a good summer that year and would have had >12hours direct sunshine/day. I returned to a flat battery. Very flat actually.
 
In Google, I searched for BTY Batteries and found, "Beware-Fake" and "Don't Buy BTY Batteries". The e-bay community said, "Stay away from defective BTY batteries".

Hmmm. When something is very very cheap and has poor quality, is it a bargain??
 
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