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I am installing a home UPS system of 1400 VA. The UPS is a 24 volt system and, based on the batteries available on the market for the UPS segment here, will need 2 x 12 volt batteries for it.

Based on what the UPS service engineer told me, and my own research on the internet, I had nearly settled on getting a 150 AH battery.

When I was talking with a battery dealer, I mentioned that my UPS was 'solar compatible', and I also planned to connect a 500 watt solar power system (PV panel + charge controller) to my home UPS in another 6+ month. He then said that I should purchase a C10 battery, as these batteries were recommended for solar power uses.

Since I didn't know what C10 or C20 ratings meant, I looked it up. Basically, all sources said that battery capacity in AH weren't meaningful as they change based on the load (current drawn). Thus, the C rating - C5, C10, C20 OR C100 - rating help with this by defining some standard hours, making it easier to compare the capacity of the battery in the market.

This is where I am getting confused, and need some clarification.

A 150 AH battery at C20, will last for 20 hours on a load of 7.5 A.
A 150 AH battery at C10 will last for 10 hours on a load of 15 A.
A 150 AH battery at C5 will last for 5 hours at a load of 30 A.

  1. In terms of backup time, which battery - C5, C10 or C20 - is better and will last longer?

  2. Why is a C10 or C5 battery considered more suitable for solar power use, than a C20?

(Be kind if my flaw / idiocy in reasoning is very obvious to you!).

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3 Answers 3

I am no expert, but since nobody else has answered I thought I could help.

I understood that the C (coloumb) rating of a battery indicated the rate of discharge that was advised. For instance, a 5C battery can be discharged at up to 5 times the rated AH of the battery. So my understanding is that a 10C 12v battery with 150 AH could have an instantaneous draw of 1500 amps on the battery. I also understand that the actual capacity of a battery does depend on the draw. For instance, I understand that lead acid batteries tend to last longer AND exhibit greater total capacity if the discharge is small (e.g., 500 milliamps). Lead acid batteries that are discharged quickly (e.g., 500 amps) tend to discharge a lot faster AND exhibit much lower total capacity. The level to which this is true depends on the battery chemistry, from what I've read. For example, if I was drawing 1 amp from your 150AH battery, I may get 150 hours of usage. On the other hand, if I was drawing 150 amps from your battery, I may only get 30 minutes of usage. The numbers were randomly picked, but you get the idea. Hopefully that cleared up your confusion. Yes, your C5, C10, and C20 batteries all offer 150AH but only under specific load conditions as you said in your question.

Not only do higher C ratings on batteries result in a higher battery cost, but, in general, high amperage charge/discharge of lead acid batteries tends to crush a battery's life significantly. This could mean that with a high enough amperage on your charge/discharge, you may only see 10 cycles to the battery before its useful life is depleted.

[1]. No battery is really better, I think. This really depends on your use case. For example, you could do one of the two extremes:

[A]. Buy just a small number of C20 batteries for your system. They're designed with high charge and discharge rates in mind. A C20 rating would guarantee that you could get the 150AH out of the battery despite a high discharge. You'd get the same total AH rating but in a smaller package. You'll probably get fewer charge/discharge cycles before the batteries need to be replaced. If you were only planning on using this solar system as a backup when the power went out (you did say UPS), then this is the better option because you would buy fewer batteries (fewer dollars). But you may need to replace the whole thing sooner.

[B]. Buy a lot more C5 batteries. They're each designed to only handle a small charge/discharge rate to give you the same total AH rating. You'd need to have a lot more of them since you're maximum demand of amps probably couldn't be met with just a few C5 rated batteries. Also, the total number of amps flowing into the batteries from the charger would need to be lower for C5 rated batteries when compared with C20. You'll probably get a lot more charge/discharge cycles out of this setup before the batteries need to be replaced. You'd probably need to consider this option if you wanted to use the battery pack for more than just a UPS. Assuming you used these batteries daily, you'd definitely want them to have a much longer lifetime/cycles than, say, option A.

In either case, the maximum discharge and the charging rate would need to be considered when you decide how many batteries and which C rating you'll need.

[2]. Lower C ratings are probably better for solar power use because most people who install a solar system plan to use it on a daily basis (unlike a UPS). Even though the lower C rating translates to a need to have more total batteries, the cost is probably a lot lower and the total lifetime in cycles is probably a lot higher.

Here's a couple examples:

Assuming you need a maximum demand of 1000 amps at 12v (12000 watts), then here are some options:

With C20 batteries rated at 150AH, the maximum amp discharge is 3000 amps. So you could definitely run the load with a single battery since 1000 amps < 3000 amps. However, your useful running time with a single battery would only be, roughly, 150/1000 * 60 = 9 minutes.

With a C5 battery rated at 150AH, the maximum amp discharge is 750 amps. So even though you probably could run the load with a single battery, it wouldn't actually give you 150 AH (and wouldn't be advised). Instead, you'd want at least 2 batteries. By spreading the load across 2 batteries, the demand from a single battery (assuming constant voltage) is only 500 amps. In this scenario, you'd be pulling (150/500 * 60) * 2 batteries = 36 minutes. In addition, the C5 battery is probably less money.

When planning this system, remember to take into consideration heat losses in your wiring which could be high depending on the gauge and length. Also remember that your inverter is only 80-95% efficient depending on how much you spend on that component. Remember that the total efficiency of what you're powering may also be affected by the quality of the sine wave (pure sine wave or the cheaper jagged/stair step kind) being output by the inverter. In the case of a computer where the power supply will simply convert the AC back to DC again, my hunch is that the efficiency of the total load will not be improved by using a pure sine wave inverter.

If I'm horribly wrong on any of these points, please somebody more knowledgable speak up! I'd be happy to know I was wrong.

Hopefully this makes some sense and is on target.

In any case, have fun and good luck with your project!

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Designed for different usage scenarios, different kinds of compromises are made on each battery type. Another parameter which may be subject to compromise would be the self-discharge rate. In the UPS supply case this is most likely irrelevant, so a compromize in this respect may well be tolerable. –  Hanno Binder Mar 27 '13 at 12:36
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I Hope the C 20 - 150 Ah battery discharge rate is not 3000 amps..

The capacity of any C rated (5, 10, 20..etc) battery is same, i.e., 150 amp- hours only.

But, if you want what type of charge and discharge of current into your battery. i.e, if you want to charge battery in 5 hours pl select C5 battery, or charge the battery in 20 hours C 20 & for solar applications C10 is the best for charge and discharge ratings.

As per my knowledge I have given the answer. for your applications use any rated battery c5, c10, c 20 there is no problem, but for solar specific c10 is appropriate.

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C5,C10,C20 all means the same meaning if it is rated as 150AH. All batteries are able to supply 150 Amps for 1 hour or 1 Ampere for 150 hours. It should follow the simple rule \$x(hours)*y(Ampers)=150\$ if it is mentioned as 150AH.

Then what is the difference between battery type,such as C5,C10,C20 etc...? The difference is only in the state of charge.

  1. A C5 battery means it should not be discharged with in 5 hours otherwise the battery life decreases
  2. A C10 battery means it should not be discharged with in 10 hours otherwise the battery life decreases
  3. A C20 battery means it should not be discharged with in 20 hours otherwise the battery life decreases

Normally all the lead acid battery's available or used for UPS are with c20 rating. because the outage of regular power is less hence operating on battery power is less,it takes long time to discharge on domestic load such as tube-lights and fans

C10 means it was used for industrial purposes and solar as the load is very high and often the load uses battery power. If we uses C20 for this purpose the load definitely draws power from with in 20 hours and keep the battery at end of discharge voltage,as by its specification the battery should not come to end of discharge voltage with in 20 hours hence the life cycle of battery decreases.

Similarly for C5, it should not come to end of discharge voltage with in 5 hours.

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protected by W5VO Mar 27 '13 at 17:29

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