OSch avatar image

MPPT 100/30 vs. 600 Ah

Is the MPPT 100/30 suitable for 5x 120Ah lead-acid batteries connected in parallel? All batteries will be of the same type and new.

If yes, what should I keep in mind?
What is your recommendation for 600Ah?

Why 5x 120Ah? Because they currently have the best price performance.

MPPT - Solar Charge Controller
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2 Answers
OSch avatar image

Hello Justin, many thanks for the detailed answer. I was very happy about it.

I understand what you are saying. For a mobile application where weight, battery size and high charge cycles are required, LFP's are the best choice. I should better describe the purpose of use:-)

It's an island solution for a garden house. The installation has a long time to load the LFAs. But when the visit comes, 300 watts should be available for about 6 hours.

The MPPT 100/30 is supplied with 2x 270 Watt panels.

2 consumer circles will be set up. 1x 12 Volt for continuous supply of alarm, monitoring and emergency light. And 1x 230 Volt via a 1.5 KW inverter for the visit.

My consideration is to try LFA's at least for the beginning. To see how long they last. (5x120Ah a. 90,-- =450,--). I hope I can count on 5 years life span for this use.

For comparison the 600Ah on LFP basis cost approx. 6000,--

I count:-): Let's assume the LFA only last one year. Then I can buy new LFA for 13 years. (6000,-- / 450,-- = 13,33 years)

Right? Or do I have a mental error?
I am very grateful for your comments.


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Justin Cook - Bay Marine Supply USA avatar image

@OSch, 30A into a 600Ah bank is pretty low... assuming your panels and sunlight are sufficient to let the controller actually provide that 30A into the batteries, you're still only at 5% or less charge rate. Now, granted, you have a maximum DOD of 50% on those batteries, so actually only 300Ah usable capacity, so you're actually sitting around 10% charge rate, but still. This may be fine for you if you don't plan much in the way of discharge or if you get so much sun that you're not worried about charging quickly, but man, I'd really think about getting something in the 60-100A range to get you charged up in a reasonable amount of time.

Again, though, that depends entirely on the panels you have as well, whether or not you have enough panels to support a 250/100 or something in that range.

Separately from all that, lol at the 120Ah FLA having the best price performance. The best price off the shelf, certainly, but considering that you're only getting a maximum of half your rated capacity as usable, and that they'll last roughly 1/6th of the rated life of a LFP... definitely not the best performance, nor the best price for the performance. I know LFP's are super expensive, but even an AGM or a Gel bank, depending on your actual needs, would last 2 to 6 times as long and give you more of your capacity in actual usable form. If you haven't already purchased your new bank, I strongly suggest you do some more comparison shopping, and consider that 1. An FLA battery is usually only rated to a maximum of 900 cycles (often less), and 2. An FLA battery can only be discharged to a maximum of 50% of its rated capacity (often less), and 3. An FLA battery will drop voltage as it discharges.

In comparison, an LFP battery can be discharged to 80% of capacity (or more) and is rated to 3000 cycles (or more) and maintains a constant voltage during discharge. If the LFP is a but pricey for you, I'd highly recommend that you look into Gel or AGM batteries, as there are several offered that get you closer to the performance and lifespan of an LFP while still staying closer to the cost of the FLA batteries.

...just my two cents...

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Hi Justin - i hear and read what you are saying quite regularly regarding FLA and lifecycle. I have FLA like the OP, mine are Trojan and this is their specs (229 A/h @C20):

I used to live on a boat and came across many boaties that had Trojan T105's (i contacted Trojan direct and they confirmed the T105RE are the same battery as the one i have copied the specs on) still going after 15 years. Obviously off grid is different application. I can't understand what people are referring to when they comment about FLA and life cycle with figures like you put forward. Just size the battery bank to your usage - i draw down between 10 & 20% on mine so don't see any reason why i won't get over 10 years out of them.

However, i do see plenty of benefits for lithium regarding being able to charge and draw at high amps - certainly very beneficial for higher latitudes and users with high amp loads.

Financially lithium just does not stack up if you know how you are going to use your batteries and that use is suitable for FLA.

(been meaning to ask this question for a long time! PS not trying to stick up for FLA - horses for courses as they say)

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