0
\$\begingroup\$

This question is strictly regarding off grid solar power setup, using PV panel, MPPT Charge controller, battery and an inverter to convert DC to AC.

Am I limited by the input charging current a lead acid battery can take?

Consider a 12V setup, where I am charging a 100AH C10 lead acid battery. Now, as per my understanding and please correct me if I'm wrong here, I should be charging the battery with 10A current. If I'm charging at a little more than 12V, say 13.5V, my power input from charge controller to battery is

13.5*10 = 135W

That kind of limits the PV panel I can add to system to somewhere around 180-200W, considering the losses, in ample daylight condition. That is one medium size panel.

So, what do I do? May be I should put two batteries in parallel or series.

Lets consider two 100AH C10 lead acid batteries in parallel. Now I can bump up input current from charge controller to battery bank to 20A. So, my input can be increased to 13.5*20=260W. It will be the same if I connect my batteries in series.

The way it seems, I am limited to using one medium sized panel per 100AH battery bank.

The charge controller I'm using is a PCM60X that supports 12/24/48V charging. At 48V, it can support upto 3.2KW. I was just wondering, how many batteries I will have to purchase to use that 3.2KW.

Of course I can simultaneously put load to the system, drawing out power during day time, while charging the battery. But I cannot keep on calibrating my charge controller for maximum charging current output for that.

Now, there is another parameter, that says maximum charging current should not be more than 0.3C, which will be around 33A for the 100AH battery. This is what I don't understand. What is the current current I should be charging my battery with? If I can go upto 33A, then it gives me a lot of head room and I can configure my MPPT charge controller accordingly.

One last question, what changes with respect to charging current if I switch to a Lithium Ferro Phosphate battery rated 100AH?

\$\endgroup\$
11
  • \$\begingroup\$ Primary limit to your system is the solar irradiation. \$\endgroup\$
    – Solar Mike
    Oct 7 '20 at 8:08
  • \$\begingroup\$ @SolarMike I live in a place where we get enough sunlight, more than we can handle, so not worried about that. \$\endgroup\$
    – de_xtr
    Oct 7 '20 at 8:14
  • \$\begingroup\$ Have you worked out how many batteries you need to store the amount of energy you need for the time you need? Usually it is a lot (for a 3.2kW controller, in a consistently sunny place, I'd be looking at about 30 x 100Ah 12V batteries), so the current in each battery ends up quite small. \$\endgroup\$
    – Jack B
    Oct 7 '20 at 9:43
  • \$\begingroup\$ @JackB 30 is like a room full of batteries. If I go for 200 AH, that's still a truck load of batteries. I was really hoping to find a way to send a higher current towards battery bank. The load on the inverter during day time would have taken care of that. My load is much higher during the day. I need not more than 400AH for the night. \$\endgroup\$
    – de_xtr
    Oct 7 '20 at 14:14
  • \$\begingroup\$ I have just noticed that some of the currents in your question don't look right. 100AH at 0.3C is 30A. 100AH at 10C is 1000A. \$\endgroup\$
    – Jack B
    Oct 7 '20 at 16:02
2
\$\begingroup\$

Supplementary:

With lead acid, for more than terrible cycle life, depth of discharge should be limited.
For really long life even with deep discharge rated cells a DOD (depth of discharge) of only 30% may be used (30 Ah available per 100 AH of capacity). More than 50% DOD is getting marginal and beyond say 70% DOD is very hard on LA cells.

LiFePO4 cells are usually rated at charge currents of C/2 or C and may be at several C (5C 10C not unknown) - this varies with manufacturer and should be readily available in data sheets. If the information is not readily available the product is suspect.

LiFePO4 almost always have far longer cycle life than LA under the same conditions. LiFePO4 is often quoted at around 2000 cycles under deep discharge use BUT this varies with manufacturer and I have seen substantially varying claims for the same cells from different suppliers.

While LiFePO4 is often said to be OK at 100% DOD I have seen warranties which require <= 70% DOD and say that at 80% DOD supplier remedial action is needed (!).

Overall LiFePO4 (compared to LA) provide lower whole of life cost per total Wh supplied.

\$\endgroup\$
2
  • \$\begingroup\$ Thanks a lot.I really want to get the LiFePO4 batteries. It's a great thing I can charge them at least at C/2 in comparison to 0.3C for the LA batteries I have. That just means I can add more solar panels to my setup and increase my charging current. However, I will have to spend some time understanding the charging profile of these batteries as my MPPTs(PCM60X) does not come with a profile for LiFePO4. Very useful information though. Much appreciated. \$\endgroup\$
    – de_xtr
    Oct 16 '20 at 17:43
  • \$\begingroup\$ Also, if OP lives in a warm location, lead acid batteries will not achieve their manufacturer reported service life. In warm locations, lead acid batteries tend to lose capacity very quickly and give disappointing performance. So I am told. \$\endgroup\$
    – mkeith
    Oct 16 '20 at 23:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.