Several questions about chargers/inverters and BMS

[JebLostInSpace]

So I'm finding that I need to run my genset a bit more than I would like. I got a new computer for work, and it's a bit of a power hog. I can run it off of my small 1000W inverter no problem, but only for a few hours before the house bank voltage gets low. I'm thinking about ways to incrementally improve my power supply to support this increased load. Also, I'm barely electrical literate, so please let me know if I'm making bad assumptions in here...

I currently have 400 Ah of AGM batteries (200 Ah usable). I would significantly struggle to house any additional batteries and keep them close enough to expand the house bank. I could do a big upgrade to Lithium and get ~400 Ah usable in the same space, but that's $$$$ so I don't really want to.

I'm also not truly convinced that I need more battery. With the full load on, I'm drawing ~10A as read by the inverter. I would think that that would give me ~20 hours on these batteries without any charging. And given that during good sunlight I'm getting ~10A in from the solar (that's in January, over the summer I regularly see 20A in from the solar), it should in fact be much longer. My batteries are ~3 years old, so maybe don't have the full 200 Ah available, but should still be at least 10 hrs I would think. However, after only a few hours (in the evening, so no solar help), I'm seeing my voltage drop below 12V.

So question 1 is: Do I really need to fire up the genset when I see voltage drop below 12V while the load is on? Or do I only need the genset when I drop below 12V while unloaded?

Related to that question: I don't have an actual BMS. I read voltage off my inverter which is supplying the power to the new computer, and that's my only indication of how much power I have. The standalone voltage readout on my instrument panel is ancient and I don't trust it. Should I invest in a proper BMS before worrying about anything else instead of using these somewhat janky methods to monitor charge?

My next idea for a near term solution is to upgrade my battery charger. I have a standalone inverter and a standalone charger, not a combo unit. My charger is a 40A charger. A little research leads me to believe that I can safely put ~25% of Capacity in Ah into AGM batteries in A. Is that correct? If so, that would mean I could upgrade to a 100A charger. I believe the 5 kW genset can supply that much power, but I'm not really sure how to double check that. I figure if I had that large of a charger, I could run the genset for much less time to maintain charge with my current batteries. I'm hoping somebody will sanity check that idea for me. Will the bigger charger really help, or does the charging profile mean that it will slow down as the batteries get closer to full charge anyway and not really save me that much time?

I also have one last question, although it's really moot at this point. I'm just confused and would like to understand. The power supply for this new computer says that it takes an input of 3.5A from a 100-240V supply. It does say it provides an output of 12A, but my understanding was that the input would be what matters for me. The output is how much current flows into the computer, but the input would be the current flowing out of the inverter. So I'm confused as to how I can be drawing 10A at the inverter when the computer is the only thing plugged in.

[rslifkin]

A proper battery monitor is a good idea, as it'll give you a good idea of how much power you're currently drawing as well as how much you've taken out of the batteries. Judging state of charge by voltage is challenging, particularly when there are loads drawing power. And not all lead-acid batteries behave quite the same in terms of SoC vs voltage or in terms of how much the voltage will drop from a given load.

The computer power supply is likely listing the maximum it can draw, not what it actually draws in typical use. And don't forget, that a 1 amp draw at 120 volts from the inverter will cause the inverter to draw 10 amps on the 12v DC side (plus a bit for inefficiency).

Generally with AGMs you can't hurt them with a too-big charger as long as it's temperature compensated. Worst case, the batteries just won't be able to accept the full output and the charger will put out less than its full capability. The only time an oversized charger would be an issue is if the batteries start getting too hot when charging quickly.

If after adding a battery monitor it's starting to look like you don't have enough battery capacity to run as long as you'd like, then I'd plan your generator runs for the times when you'd be drawing the most power (that way you can get some of that directly from the generator vs having to pull it out of the batteries and put it back later).

If you've got enough battery but the solar isn't able to get them fully charged most days (not producing enough power relative to what you use), then a generator run in the morning is the best time (as the batteries will be lower, so you can put some power into them quickly, then shut the generator down and let the solar finish the slow part of the charging process).


FWIW, I've got an 80A charger (in my inverter) on my 415ah AGM house bank. I'd say it's reasonably sized and I'd have no reason to make it smaller. At some point I'd like to double my house bank and at that point I wouldn't mind a bigger charger.

[ranger58sb]

Originally Posted by JebLostInSpace

I currently have 400 Ah of AGM batteries (200 Ah usable). I would significantly struggle to house any additional batteries and keep them close enough to expand the house bank. I could do a big upgrade to Lithium and get ~400 Ah usable in the same space, but that's $$$$ so I don't really want to.

How old are the current batteries, what BCI group, what brand?

-Chris

[JebLostInSpace]

Originally Posted by ranger58sb

How old are the current batteries, what BCI group, what brand?

-Chris

The batteries are 3 years old from West Marine, I'm not sure on the BCI group. (I also can't fathom why the PO bought batteries from West Marine, but there you have it.)

[JebLostInSpace]

Originally Posted by rslifkin

And don't forget, that a 1 amp draw at 120 volts from the inverter will cause the inverter to draw 10 amps on the 12v DC side (plus a bit for inefficiency).

Ahhhhh that explains it! I was thinking 3.5A was 3.5A. But for the inverter to supply 3.5A at 120VAC, it's drawing much more than that from the batteries at 12VDC. I knew I had to be missing something!

[rslifkin]

The West Marine batteries are a little expensive, but they're made by East Penn. Basically rebranded Dekas. So they're generally good batteries, although those AGMs don't have the best deep cycling performance. The gels are quite good, and any of the WM batteries make good starting batteries.

Depending on the life they've lived, it's possible they're getting to end of life after 3 years. When it comes to considering replacing them, measure the space you have for batteries including height. That'll give an idea of what can fit in there going forward.

[JebLostInSpace]

I also forgot to ask another question about the chargers... If I wanted 100A charging capability. Could I just add a 60A charger and run it at the same time as my 40A charger? Will the two be able to run at the same time without messing up the charging profiles? Will I run into trouble trying to wire the second one in? From what I've seen so far, a 60A charger is substantially less expensive than a 100A charger, so it would be nice to go that route if I do decide to upgrade the charger.

[rslifkin]

Originally Posted by JebLostInSpace

I also forgot to ask another question about the chargers... If I wanted 100A charging capability. Could I just add a 60A charger and run it at the same time as my 40A charger? Will the two be able to run at the same time without messing up the charging profiles? Will I run into trouble trying to wire the second one in? From what I've seen so far, a 60A charger is substantially less expensive than a 100A charger, so it would be nice to go that route if I do decide to upgrade the charger.

In general, yes, you can do that. As far as getting optimal charging in that configuration, it'll depend on how the chargers decide when to end the absorption phase of charging.



The other option would be to swap to a bigger inverter/charger and let that take care of all of that and just use the existing charger to take care of the starting battery.

[ranger58sb]

Originally Posted by JebLostInSpace

The batteries are 3 years old from West Marine, I'm not sure on the BCI group. (I also can't fathom why the PO bought batteries from West Marine, but there you have it.)

So middle-of-the-road AGMs, OK dual-purpose batteries. Lifespan depends on how they've been treated. I don't remember whether East Penn publishes a 1000-cycle graph for those, but if they don't, they're not really "deep cycle" batteries.

Lifeline and Odyssey both publish 1000-cycle graphs. Lab tests, so not perfectly like real world, but at least slightly useful for comparison purposes.

With your current batteries, fully charge, take them off the charger for at least 4-6 hours, read voltage then. That might give you at least a quick-'n'-dirty reading on how they're doing. Could just be age. OTOH, if you still have a good voltage reading then, something else on your ship might be worth checking.

As RSL says, when it comes to measuring your space, consider height too. Some of the taller 6V batteries can sometimes fit within the same horizontal footprint and still add significant capacity.

If you've got 4 batteries in that 400-Ah bank, I'd guess they might be BCI Group 31. 4x 6V L16s, for example, might come close to fitting in the same space (They're taller), and that could get you up to ~600-Ah (IIRC) with no other changes.

-Chris

[rslifkin]

Originally Posted by ranger58sb

So middle-of-the-road AGMs, OK dual-purpose batteries. Lifespan depends on how they've been treated. I don't remember whether East Penn publishes a 1000-cycle graph for those, but if they don't, they're not really "deep cycle" batteries.

Lifeline and Odyssey both publish 1000-cycle graphs. Lab tests, so not perfectly like real world, but at least slightly useful for comparison purposes.

With your current batteries, fully charge, take them off the charger for at least 4-6 hours, read voltage then. That might give you at least a quick-'n'-dirty reading on how they're doing. Could just be age. OTOH, if you still have a good voltage reading then, something else on your ship might be worth checking.

As RSL says, when it comes to measuring your space, consider height too. Some of the taller 6V batteries can sometimes fit within the same horizontal footprint and still add significant capacity.

If you've got 4 batteries in that 400-Ah bank, I'd guess they might be BCI Group 31. 4x 6V L16s, for example, might come close to fitting in the same space (They're taller), and that could get you up to ~600-Ah (IIRC) with no other changes.

-Chris

L16s are usually around 400 - 420ah each, so 4 of them would provide 800+ ah at 12 volts (my house bank is currently 2x L16 and if I move where they're located I can easily make it 4x L16). GC2s are the same footprint but not as tall and usually around 260ah. So 4 of those would be ~520ah.

[wingssail]

Originally Posted by JebLostInSpace

...I also have one last question, although it's really moot at this point. I'm just confused and would like to understand. The power supply for this new computer says that it takes an input of 3.5A from a 100-240V supply. It does say it provides an output of 12A, but my understanding was that the input would be what matters for me. The output is how much current flows into the computer, but the input would be the current flowing out of the inverter. So I'm confused as to how I can be drawing 10A at the inverter when the computer is the only thing plugged in.

OK, if your computer says it uses 3.5 amps at 120 volts, that, if true, would be over 400 watts. Coming out of the inverter this would require about 35 amps from your battery. So the actual usage by the computer is probably unclear but it probably is a bit more than what you are thinking.

Next, the 12A output is probably an outlet on the computer into which you can plug something, so if nothing is plugged into that outlet, it does not affect your calculations.

Next, you are focused on the inverter, but you may have other loads, such as lights, or refrigeration. If your normal load at night is 10 amps the inverter/computer simply adds to that, you could easily explain how you use your 200 amp capacity is short order.

The first thing I would recommend is a good battery monitor, or even just a simple shunt/ammeter which can measure all your loads. This will tell you how much you are using and help you understand why your batteries run down so quickly.

Finally, none of your system components can be considered separately. More charging capacity without more battery capacity won't solve your basic problem (batteries run down too quickly). Supporting a "hog" cpmputer and other loads on battery power will require more capacity, better monitoring capability, and more charging capacity.

[SV Confianza]

Originally Posted by JebLostInSpace

Ahhhhh that explains it! I was thinking 3.5A was 3.5A. But for the inverter to supply 3.5A at 120VAC, it's drawing much more than that from the batteries at 12VDC. I knew I had to be missing something!

Watts = Volts * Amps so to go from 120v to 12v you convert to watts first and then divide by volts.

If you’re going to need household type power for beasty laptops, you’ll probably need more capacity (and maybe some reserve if it’s not sunny). I would suggest doubling your capacity from what you have at least. Or do the lithium conversion. But lithium is much more than just expensive batteries (new alternators, regulators, maybe solar controller).

Lithium has a ton of advantages in terms of charging speed and depth of discharge. If you’re planning on working from the boat a lot, it’s worth considering.

[wholybee]

You really need a good meter to know exactly what the power usage of this computer is. I am deducing from the above posts that it is probably around 10A at 12V. Yes, that should give you around 20hours of use with a 400Ah AGM bank.

A couple considerations though. The inverter will turn off at a certain voltage. And with a load on the battery, it the voltage might sag below that before you get 200Ah out of them. So, you might have battery power left, just not enough to supply 10 amps.

Second, AGMs are probably a poor choice for this application. Several posters on here suggest they not be used on boats at all. The problem is that AGM batteries MUST be charged to 100% daily. To get to 100% (not 95% and not 98%, but 100%) would require running your generator for about 5 hours every day. If you do not do this, you will quickly damage them, and your 400Ah bank turns into a 100Ah bank. They are 3 years old. If you have not been charging them fully, it is a safe bet they need replacing.

You would need to test them to know the condition. To do this, you need a good SOC meter, and a way to provide a constant 20Amp load. You would charge them all the way up to 100%, then connect the 20 Amp load, and discharge to 10.5V (0%). That should take 20 hours. If it takes less, do some math to calculate the actual Ah the batteries have.

Flooded batteries are better, as they do not need to be charged to 100% every day. Still about once per week though. Lithium, in addition to being able to use 100% of the battery, never need to be charged to 100%. That is the main reason they are so great on boats with solar. Lithium (some of them anyway) are also smaller. You should be able to fit 600Ah or more of lithium in the same space as 400Ah of lead. You could go from 200Ah usable to nearly 600Ah usable battery in the same space.

But if you can't do Lithium now, flooded are the cheapest bang for your buck, and work better than AGM for solar and PSOC (partial state of charge).

[ranger58sb]

Originally Posted by rslifkin

L16s are usually around 400 - 420ah each, so 4 of them would provide 800+ ah at 12 volts (my house bank is currently 2x L16 and if I move where they're located I can easily make it 4x L16). GC2s are the same footprint but not as tall and usually around 260ah. So 4 of those would be ~520ah.

Ah. I must have been mis-remembering based on some of the taller 6Vs that fall in between GC2s and L16s.

IIRC, the Lifeline GC2 equivalents we had gave ~220-Ah/pair, ~440-Ah total in our case.

-Chris

[JebLostInSpace]

Thanks to everyone for the help so far!

I think I am going to go ahead and order a Renogy battery monitor. I might also pick up this RecPro 100A charger I found for RVs that's surprisingly cheap. Anyone have experience with them? The reviews are a little mixed, but I'm thinking I can install it independent of the current 40A charger and switch only one on at a time. Then, I can monitor the performance of the cheapo RecPro, and if it turns out to be problematic, I just rip it out no harm no foul. And if I go back on dock power for whatever reason, I can easily use the smaller 40A charger...

After I get those two pieces of the puzzle going, I'll use the new battery monitor to judge how my power storage is holding up. Then I'll decide whether I need to replace batteries and upgrade storage. I've been perusing 6V flooded golf cart batteries. Sadly, almost all of them are too tall to fit where my current house bank lives. When the time does come to do batteries, I guess I'll either do the big lithium upgrade, move the house bank to a new location and re-route a bunch of wiring (blerg), or continue living with about the same capacity... Or maybe build a custom floor panel that "bumps" up to make room for the taller batteries lol. I'm sure nobody would ever trip on that in the night :P