question

Christo avatar image

Max discharge current for AGM Battery bank

Hi All

I have a 6ooAh AGM batt bank on a sailing boat (3 x 200Ah)

What is the max safe current drain for said bank, especially with longevity in mind?

Reason:

I have a 1,6kW 230V AC watermaker and would like to run it off the batt bank through an inverter for about 30min at a time. (and some other power tools on the odd occasion)

I do have a generator so that watermaker can be run for longer periods but for 30min (making 50L of water), it would be very convenient to not use the generator, not to mention saving petrol, fumes, and noise.

I have 640W solar and 300W wind gen which should be more than enough to deal with the discharge, I believe, on a sunny day.

So, is there a rule of thumb for a max safe discharge current for (AGM in my case) Lead Acid Batteries?

My gut feeling is that 300A for an hour on a 600Ah bank should be safe. But then my 2nd gut will freak out when it sees 200A of discharge on the BMV.. Even 100A is a hell of a lot in my mind.

Regards

AGM Batteryampsdischarge level
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7 Answers
Bill Nichols avatar image

This thread is a bit old but I figured I'd put in my own $.02.

The service life of a deep cycle battery is measured in discharge cycles. This is usally promised by the manufacturer of the battery.

Each 100ah promised by your battery bank is at a 20 hourly rate at 5 amps. The amp-hours drops the greater the current draw. At 5 hours on a 100 a-h battery for example you might get 82a-h at 16 amps. The manufacturer will give you a table on this. No idea how many hours you can expect at 100 amps more. But don't expect 6 hours out of your 600 amp bank.

Take good care of your battery including the correct charge controller, not draining them too deep, and you will achieve the best# of discharge cycles.

A 12v watermaker might be more appropriate for the use case being discussed.

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JohnC avatar image

Hi Christo. Yeh, you knew you were pushing the envelope, and largely got through it. If the batts can hold 12.5V I wouldn't be too concerned. Might depend somewhat on whether they're deep cycle designed or not. If they have a 'C rating' they'll *usually* be designed for auto use, and to stay fully charged. Of course then they're still expected to deliver v high start currents.

I have a great confidence in the Multi, and consider them bulletproof. But warnings are warnings (whatever for), and maybe it's saying "Hey I can do this, but I'm gonna shut down if you keep it up"? The fans on my 5kVA seem to wind up proportional to the load. Maybe it's not a temperature thing??

Likely you'll eventually tire of close monitoring, and will revert to the genny. Or half-genny-half-batts. You're already aware there's a setting to limit input to the Multi, but there's also a minimum you can apply, depending on model. That shouldn't impinge on what you do, but just be aware it's there.

Ie. Start the genny, after a little time it will switch through and start charging the batts and covering loads. Switch on the water maker, then change the genny to 'economy' mode. That mode won't like massively changing loads, but it will save fuel and the genny won't be screaming it's heart out.

As you know, boats are for 'mucking about on'. Have fun. Fair winds.. :)

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Christo avatar image

Hi @JohnC

I just installed the water maker (no generator yet, still shopping)

I ran the water maker around mid day with fairly good sun on solar panels.

Generally I easily see 30A or so charging. With that in mind, the water maker mostly showed a current draw of -80 to -90A on the BMV.

The inverter fan ran from the beginning. I was going to run the water maker for 15 min but 15sec before 15 min the red alarm light came on on the MultiPlus Compact even though it kept on running. Naturally I stopped the water maker.

My Batt bank voltage never dropped below 12,50V and the batteries and cables stayed cool. Do you think that this impact on the batt bank is minimal or am I shortening it's life? (15min - 28Ah drain (600Ah bank))

I realise that the inverter is running at it's limit; do you think unhealthy so, or could I run it like this indefinitely, say, for 14 min (before the red light came on) at a time?

I will get the Ryobi 2000W inverter generator for long term water making btw.. (will look into reprogramming the inverter to "take" less from the 230V AC source)

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Christo avatar image

And @JohnC, another question.

You mentioned that the Victron Multiples Compact might not accept power from any genny... Why is this? I can imagine that the only problems could be the quality of the sine wave, frequency, or voltage. I mean, what else is there?

I am looking at buying a Ryobi 2000W inverter Generator RG2000I. Do you think that my Multi would be happy with that? I do so want them to be happy..

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Re your batts, see this link (probably a Victron competitor, but I'm ambidextrous - clicking with my left hand now): http://www.smartgauge.co.uk/batt_con.html

Re the genny. A difficulty some may never experience, but if you do can be a right pain. The Multi is fairly picky about dirty power, and a lot of gennies can deliver that. Not a feature to be criticised, just by design. It can be worked around though, so not a gamekiller even if you came across it.

Personal experience, I've found inverter gennies to be fine with a Multi, and that Ryobi should be ok. It's at the lower end of a suitable size though, I'd go a bit larger, but that's just me. Nice if you could borrow one to test..

Feels bare without another link, so: https://www.victronenergy.com/live/multiplus_faq

Thanks for sharing @JohnC! This is a very good explanation, even i clicked it with my left foot wearing socks ;o)

Thanks for the link John, that explains it very well. I've been wanting to devise a practical demonstration of this interconnection method for a while, but it takes some time to do. Maybe when I'm not so busy I can do it.

Christo avatar image

Hi @JohnC

You mention:

3x batts in parallel, 12V, assumed. Firstly, multiple paralleled batts may suffer from one delivering more than it's neighbours. May not apply with nice new ones, but with a little age on em, could kill. Just a little something to watch for.

I do indeed have 3 12V batts in parallel. That means all the +'s connected and the -'s connected but the + and - to the load come off opposing ends of the bank (pic) and not, for example, off a single battery.

(My thinking is that I wanted to keep the conductor lengths symmetrical and equal for all three batteries, meaning equal loading on all of them. Would that minimise your concern?)


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Christo avatar image

Hi @JohnC

I should have said that I'd do this only when the sun is high on a sunny day. 640W Solar = 53A at 12V theoretically. So I'd be charging the batt bank with say at least more than 30A, let's call it 30A. That makes the load on the batt bank @ 1600W; -133A + 30A = -103A

If I run the engine (alternator) it is -133A + 30A + 50A = -53A but then I might as well use the generator and actually charge the batt bank while making water (2kW generator)

By the way, I'm using a Victron Multiples Compact inverter/charger. I'm thinking to run all my AC loads with the generator through the Multiples Compact. Am I right that the inverter would directly switch the generator supply to the watermaker with 2 advantages to my understanding (not 100% sure if I understand it correctly):

  1. If the AC load is not too big, my batt would be charged at the same time
  2. If the AC load is too big (like a high starting current) the inverter could boost the generator output from the batt bank

All in all, it might just be better to use the generator, is the feeling I'm starting to get. That way the batteries will simply last longer.

Thanks for the help

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Yeh, your Multi would work just like you envisage. Providing it will accept your genny power (separate issue).

Have a go either way. At the very least it will help you get to know your kit & it's limitations. You won't hurt anything trying with a watchful eye on the batts.

JohnC avatar image

Hi Christo. This a little like playing 'pin the tail on the donkey'. But (heehaw) i'll have a go to 'kick' you away..

3x batts in parallel, 12V, assumed. Firstly, multiple paralleled batts may suffer from one delivering more than it's neighbours. May not apply with nice new ones, but with a little age on em, could kill. Just a little something to watch for.

If you draw too much, the V will soon show it. Ideally you'd try with the batts charged, at least a fair ways into Absorb cycle.

Let's say your load less generation is ~1000W. Round figures, 80A. So 14% A vs Ah, and that's possibly a stretch for AGM. The V will confirm it though under load. Below 12.5V with full charge aboard I'd wonder hard. Below 12V I'd turn it off.

Just me & my personal opinions though. Others may differ, welcome to the paddock.. :)

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