Question about capabilities of a rechargeable battery

[Sajjadkhan]

Re: Rechargaeble Battery

ckshivaram is right, this is a lithium-ion battery and cant be charged at constant current. Rest your answer is within your provided data. Use 7.2-7.5V to charge the battery. The battery will be charged when battery voltage equals to applied voltage. at this stage your voltage must be reduced to standby voltage as 6.75-6.9V so that it cant be discharged via environmental conditions. battery will draw current according to its need. when fully discharged it will draw 1.2 amps. So your combination of solar cells must provide at least 1.2 Amps otherwise it will be a slow charging.
But be aware these are extremely sensitive batteries and even a little delay in switching the voltage down to standby after a full charge will over heat the batteries, ruins them and they often explodes. If you do not have a mobility issue then use sealed lead acid batteries instead, they have high tolerance to high voltages.

 

[Krithiga]

Re: Rechargaeble Battery

I have another doubt ...since i use solar celll/panel as power source and the charger needs around 9-10V as input ...te size of the cell/panel wud be big..wat my doubt is
1. Can we convert te voltage from solar cell ..i mean 2V, required amps (frm solar cell) to 10V through IC's ..then giving it to the charging circuit which produces the necessary voltage and current for charging ?? will this work out ??

 

[FvM]

Basically, a boost converter can achieve this. For maximum efficiency, it would directly connect to the battery rather than dissipating power in an additional charger circuit. But I wonder, if a small 2V solar cell will supply the intended charging power.

 

[Krithiga]

Thanks alot FvM...i got it U need 2 consider law of conservation of energy too..

 

[Sajjadkhan]

2v is really small for boost converter i think. Consider LM2577 but it requires 3.5V min,5V recommended t0 get 800mA as circuit it the datasheet says. Anyways it would be a good IC for charging batteries.
How much solar cells do u have and the current and voltage rating per cell?

 

[Amir.AJ]

Hi everyone

I just wanna ask some of the experts here, whether it's is OK for me to charge a lithium-ion battery (let's say a 2Ah), with a constant current of 50mA (constant current stage) and a float voltage of 4.1V (constant voltage)?

The problem here is that normally, one would charge the battery at 1C rate at the constant current stage, but I'm just wondering would it work if we charge it with a really small charging current (for example: using photovoltaic source, solar, etc)

Despite the slow charging rate (longer time of course), would this work and would there be any other problems?

 

[Sajjadkhan]

Yes you can for this small current but whats the logic. It would take 40 Hrs to charge the battery. If you charge it on the rated voltage, it would take less than 2 Hrs.
Let me tell you why constant current is recommended. Suppose you have attached two 12V batteries in series B1 and B2. Now suppose B1 voltage is 10.8 and B2 voltage is 11.2. Now you charge them at 24V and charge is to be terminated when B1 + B2 = 24v i.e. sum of both batteries voltages equals to the applied voltage. since b2 is greater, it will always be greater when the charge is terminated B2 will be at say 12.4V and b1 will be at 11.6V. your charger will be fooled assuming the batteries are charged but in fact b1 is not properly charged. So here comes the constant current mode. As current is constant, it will be same through both batteries so b2 voltage will not increase until b1 equals to b2. So constant current is required to bring all the cells to same potential level. Your battery pack may contain multiple cells but since they are intact their voltages would be the same.

So charge your battery at rated voltage and terminate the charge when charged. No float charging. I have see batteries exploding and one (SLA) battery exploded to my face which even can tolerate high voltages. But Lithium-ion batteries can't tolerate voltages when charged. At your voltages they will be damaged and if the are tightly packed, they will explode. So don't take risk.

 

[Amir.AJ]

Thanks Sajjadkhan for your good explanation. Now I know how important the constant current stage is.

The small charging current is due to the fact that I use a photovoltaic source and instead of using a low-capacity Li-Ion battery, I just wanna know what would happen if a high-capacity battery is in use, and charged with a low constant current. As i thought, it would definitely takes longer to fully charge it.

I have another question, could you tell me if its possible for us to float charge a Li-Ion? (Would likely be the third stage of charging an SLA battery, but I wanna know the effect of this method onto a Li-ion battery, for example, at a 4.1V float voltage, with a low current at around 20-50mA). Normally a charger would terminate itself after the battery is fully charged, but an SLA is capable of withstanding a float voltage charging; is a Li-ion capable of such charging method?

Thanks to Sajjadkhan and to everyone

 

[Sajjadkhan]

Just like earlier i explained that you can not float charge Li-ion batteries as it will degrade its life. Example I have my Laptop Hp Pavilion DV6 series which usually comes with Li-ion battery, all laptops and cell phones do. Sorry to say that it has poor method of battery charging which goes to float mode when battery is charged. So after six months my battery health started to degrade and 2 months after that my battery wouldn't charged and it cost me a new one.

Now I think you may know that leaving the battery open will discharge it due to environmental conditions but the discharge rate is very slow. For SLA batteries its 5% and for Li-ion is 30% over 1 month. And thats why you are interesting in float charging to keep it active.

As you can't float charge, here is another and effective method. Search for SOC (State of charge) table for Li-ion batteries. Note at what voltage level battery is 90% charged. Then you just need to build a comparator circuit (with Hysteresis) which terminates charging at batteries rated voltage and starts it when battery voltage reaches to 90% of its charge level. If a battery is expensive then search for its safe charging temperature and attach a temperature sensor to it.


Steps of charging:
Step1:
OK if your Li-ion battery have more then one cell, you can constant current charge up to 0.8C and your charger MUST MUST (2 times )switch to constant voltage mode (step 2) when batteries are 90% charge and that voltage will be you rated battery voltage and when your battery reaches to rated voltage then terminate the charge. (step 3) After that your comparators will come in to play.

If you need maximum run time from your batteries you need to amend step2 i.e. instead of taking voltage as you charge termination use current. When your current gets < 3% of rated battery current then terminated the charge but in this case do track the temperature.