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moose4621 avatar image
moose4621 asked

Replacement for flooded lead acid batteries.

I asked this question over on the whirlpool forum and got some great answers, one of which strongly recommended Victron inverter and BYD batteries. He also suggested I ask the same question here so.....

"I have a 48v stand alone system with 3.6Kw solar, 3.5 kva latronics inverter, Plasmatronics PL60 charge controller and Raylite 1380ah flooded lead acid batteries.

The batteries need replacing.

I have managed to get 13 good years from them so I am happy with that.

But now, with a budget of around AU15 – 16k, what do I do?
I have had companies tell me to throw everything away and start again with a relatively small lithium setup.
I have also been told I can integrate lithium into the current system with significantly smaller capacity.
I have been quoted on several flooded and gel packs all of which seems to be at the upper end of the available budget.

Cycle life seems to be all over the shop depending on who you are asking and each outlet will tell you that their chemistry or construction or configuration is "the only way to go".

The amount of information/misinformation is intimidating and I don't quite know who to believe so I am turning to the well experienced people here to offer there suggestions and reasoning.

The replacement of these batteries is of course, time critical as the gennie is getting a flogging at the moment.

Many thanks in advance,"

Lithium Batterysolarsystem designlead-acid
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HI,

I have a couple of questions, all important for lithium...

1. What kind of average depth of discharge (DoD in %) did you make to have them last these years?

2. What was / is the temperature interval felt by the batteries in the storage facility?


With 1380Ah /48V, this is over 66 kWh, if we make the usual 50% DoD reference, it still is a massive 33kWh. Now, if you cycle up to 80% in lithium, it is a very expensive lithium bank.

But, if you tell us you don't need 33kWh to start with, you can get away with a less expensive bank.

One indication this might be true is the solar array... it seems to me a slightly on lower side for such a battery. Did you manage to charge to 100% every day? Maybe with lower DoDs you can, so lets work on that value. Do you have the estimation/ measures for the load consumption?

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It seems few people are allowing for more than one day of supply.

Yes 1380ah is a large pack but at the time of purchase I was told to stay above 30% DOD and 20% is even better.

At 30%, that leaves me with just 2 days of autonomy @ 200a/h day.

I have no problem reaching full charge every morning in fine weather. They usually hit float by 10am. A different story of course following bad weather, but still more than acceptable solar performance.


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4 Answers
kai avatar image
kai answered ·

It sounds like you're looking at a land based / fixed installation. For a given capacity (figure this bit out without committing to a battery technology), lead acid will have a much lower upfront cost, if you're not being ripped off in quotes.

LiFePo's great if you're in a mobile environment because of weight savings.

LiFePo's ability to repeatedly draw down to a much lower depth of discharge and longer cycle life is I think very dependent on manufactured quality, system design and just plain luck. I can't recall seeing evidence that gives me the warm and fuzzies on longevity aside from claims in datasheets (it probably exists, but is locked up as very expensive IP).

LiFePo's cost savings over LA in the longer term is highly dependent on the above playing out in your favour.

Additionally you will need to get used to the Battery Management System attached to the LiFePo (unless your installer keeps an eye on it for you).

On balance, at this point in time I'd say look harder at lead acid unless you're prepared to deal with new technology and everything that comes with it. Its a risk-reward thing.


(Fyi, I'm using LiFePo myself)


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graham avatar image
graham answered ·

moose4621 Some basics about lead acids & lithiums the deeper they are discharged each cycle the shorter their life is. Life is defined as being the point at which the battery holds only 80% of it original capacity when fully charged.

Victron Lithium will do 5000 cycles discharged to 50% of capacity and discharged to 80% of capacity they will do 2500 cycles.

I think the biggest advantage that lithiums will have for you is that they make far better use of the solar input, charge in about a third of the time to the average lead acid if on occasion you have to revert to a generator etc for charging.

Yes it should be quite easy to integrate lithiums into your system it is something I do all the time albeit for much smaller system on boats.

In order to help we need to know what is the daily usage, the daily input from the solar and any other charging source.

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moose4621 avatar image
moose4621 answered ·

Thanks for the responses people. I appreciate it.

The DOD of the lead acids were approximately 15% for the last few years, but for the first few years of their life it was more like %30 or even 40% at times.

At the moment the batteries are easily reaching full charge by am 10am on sunny days. The generator usually never kicks in. I tend to use it for high ac loads like large welding jobs.

Doing a straight lead acid battery replacement is certainly the easiest way to go for me, but is it the best option for the money.
I do not have an aversion to new technology as long as it is proven.

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Hi Moose,

"The DOD of the lead acids were approximately 15% for the last few years"

That works out to be about 10.5kwh, or about 200ah@48v. A 400ah lithium bank would be equivalent @ 50% DOD.

The only other thing to note would be that your solar array might be close to average demand, could be an idea to plan for some upgrade there.

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That would leave me with very little in reserve for bad weather, wouldn't it? What do most people allow in terms of "days of autonomy"?

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A larger array will help with both of those.

Panels are cheaper than batteries.

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moose4621 avatar image
moose4621 answered ·

@kai it is a land based system, yes.

I have been hearing a lot about nickel iron batteries too. Seems the maintenance can be high though.

Arrrgh, my head hurts!

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You're getting a number of competing opinions and I don't think its going to change for battery technology regardless of the forum.

There's various techniques you can use to sort out what you need from all the noise, they all largely boil down to making a list of pros and cons for each option (LA, LiFePo, etc etc).

The only thing I would add to that is to factor in the risks (to safety, to wallet, etc) and your confidence in the information.


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Yes, I think you are right. There seems to be several correct answers to this problem.

I wish it was clear cut.

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