Pls HELP :) Temp charging LiFePo4 battery bank with generator

Wishing4freedom

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Mar 12, 2022
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New here, hello!
I have a camper, not rv
I'm trying to start off with two 12v 200ah LiFePo4 batteries in parallel, temporarily charging by my generator through the shore power cord (30amp) and use a ~5000w PSW inverter (so that for the short time I'm cooking, I can run all my appliances) and it's looking like it's going to be cheaper for me to get a separate charger, and maybe better for what I'm trying to do ?? I'm wanting to start my generator up when my batteries get a DOD of 25-45% and charge them to 75%.

So what I'm asking is, do I need an automatic transfer switch or a regular one? Where does it wire to? If I need to use power while they're charging, will it have me feed from the batteries or the generator? And is there anything else I need if I'm not getting an inverter/charger combo? Any fuses?

In the near future I plan to get a couple panels and charge controller going and by mid fall have another battery

Please help 🙏
 

rmaddy

Full-time Solar-powered Trailer Life
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A 5000W 12V inverter? That's crazy. That's up to 500A from the batteries. That requires ridiculous wiring and fusing. And can your batteries each provide 250A of discharge current? Not many LiFePO₄ BMSes can support that much current. An inverter that large should be done with 48V, not 12V. Then you only have to deal with 125A of total discharge current.

I'm wanting to start my generator up when my batteries get a DOD of 25-45% and charge them to 75%.
Why do you only wish to use 30%-50% of your batteries? You can easily use 90% of a LiFePO₄ battery without issue.
 

time2roll

Photon Sorcerer
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Connect the charger to the battery. Plug the charger into the running generator. Unplug as the battery reaches 13.6 to 13.8 volts.
Repeat as the battery drops to about 13.0 volts.
 

jimcalf

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From what you're describing, you'd be better off with just getting an inverter charger. Something like a Multiplus 12/3000/120 would allow you to set the input current to support the max your generator can offer (most don't provide a full 30A), pass through the 120V needed for your appliances and use what current is not needed for AC appliances to charge your batteries at the same time. And it's all automatic. For the moments when you want to run the microwave for a few minutes, with the AC running, the inverter will actually supplement the shore power to meet the needs of the AC load, and then switch back to charging when the extra demand of the microwave stops. I've never found a system that can manage AC loads and charging simultaneously as well as a "hybrid" inverter/charger. Also, buying a charger that can put 120 amps into your batteries, like the Victron Multiplus, is expensive. I tried to replicate the inverter and charger capacities, plus the needed ATS, and the cost was almost the same as just buying the much more sophisticated Multiplus with way less functionality. It might look expensive, but the Multiplus 12/3000/120 is a very cost effective for $1,285. If you did go with a 3000VA Multiplus, you really need more battery. At least 300ah to meet the potential current draw of 250A.
 

Wishing4freedom

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Mar 12, 2022
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1. Thanks for the replies!

I wish to keep DOD at 25-75% to drastically increase the life cycle of the batteries. Going with 24v inverter would require an even amount of batteries and after getting this all set up(the 2 12v 200ah batteries, inverter, and charger, whether it's in the inverter or not, and new panel box), I'm focusing my money on getting solar panels going for the summer (to utilize the sun time) and then save money to get the third battery going in fall for longer ah in winter as I don't want to be hooked up anywhere. (We will not be in snowy, cold weather, just chilly with less sun) and then we probably will not have more money to spend on the system until towards spring, which will probably be more solar. Until income tax 😏

I'm trying to set myself up with the 5000w inverter so that I end up with what I want while I can't buy it all at once. The only time I'd be using the 5000w is when I'm cooking and I need to do a quick burst in the microwave (1500w) (15 seconds to 3 1/2 min) while I'm running the heater/ac (1500w) and cooking on the griddle or in the countertop oven (both 1500w) and then my fridge is 345/750(surge). Or if I want to use my oven, ac, and griddle at the same time (4500w at 12v), even if it's for 30 min.

After getting my batteries and new inverter and charger set up and running(needing to leave where I'm at), then I'll be installng my solar panels (24v, trying to start with ~800W, this is why we will be supplementing with the generator until we can accommodate more solar) We just need to get out of where we're parked and on the road and able to leave shore power (no batteries hooked up right now, and can't run on full generator use either, because that's a lot of gas to have it running 247 for the fridge.)

What @jimcalf is describing with the being able to use power directly from the generator while it's charging the batteries is what I'm going for. Is that what an automatic transfer switch is?
@rmaddy
 
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Hedges

I See Electromagnetic Fields!
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I don't think you can determine 25% or 75% SoC of a lithium battery except by tracking amp-hours in/out starting from full. The knee of the curve is how you recognize full or empty.

Cycled to use 80% or 90% of capacity, between the knees, claimed cycle life is typically 3000 cycles so almost a decade of daily use. And that only if drawn down that much. How many watt-hours do you expect to use per day? Per night? Once you set up some PV panels, the battery may just provide surge when needed, never get drawn down much.

So I would think just use it, don't worry about impact of cycles on life. But avoid charging when battery temperature is at freezing, and avoid keeping fully charged when very hot.

Any reason for a 12V system, rather than 24V? Half as much current at 24V.

2 x 12V x 200Ah = 4800 Wh battery.
800W x 5.5 hours effective sun = 4400 Wh average from PV panels, if tilted according to latitude (for typical location.) Maybe 1600 Wh in winter.
800W / 4800 Wh = 0.16C, a mild charge rate. Probably 3x as much panels would be OK for typical lithium battery 0.5C charge rate (only if moderate temperature)
 

HRTKD

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From what you're describing, you'd be better off with just getting an inverter charger. Something like a Multiplus 12/3000/120 would allow you to set the input current to support the max your generator can offer (most don't provide a full 30A), pass through the 120V needed for your appliances and use what current is not needed for AC appliances to charge your batteries at the same time. And it's all automatic. For the moments when you want to run the microwave for a few minutes, with the AC running, the inverter will actually supplement the shore power to meet the needs of the AC load, and then switch back to charging when the extra demand of the microwave stops. I've never found a system that can manage AC loads and charging simultaneously as well as a "hybrid" inverter/charger. Also, buying a charger that can put 120 amps into your batteries, like the Victron Multiplus, is expensive. I tried to replicate the inverter and charger capacities, plus the needed ATS, and the cost was almost the same as just buying the much more sophisticated Multiplus with way less functionality. It might look expensive, but the Multiplus 12/3000/120 is a very cost effective for $1,285. If you did go with a 3000VA Multiplus, you really need more battery. At least 300ah to meet the potential current draw of 250A.

This is a great answer. It's exactly what I did in my RV trailer.

My 1500 watt rated microwave pulls almost exactly 1500 watts from my inverter. The Multiplus is rated for 2400 watts so that gives me plenty of headroom to run other small AC loads while the microwave is running.

Some compromises are necessary in a mobile environment. You simply may not be able to run everything you want at the same time. I know that the coffee gets made first, then I can crank up the microwave to warm up my breakfast burrito. I huge inverter (5000 watts is huge) doesn't mean it will all work. There may be other choke points within your system. Are those batteries going to be happy having 200+ amps pulled? They should since they're 200 Ah batteries, but you didn't mention the brand and not all batteries and BMS are created equal.

An integrated inverter/charger is so much easier to deal with. As stated already, it has the Automatic Transfer Switch built in. There is less wiring involved with that single unit versus separate inverter and charger devices.
 

time2roll

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I wish to keep DOD at 25-75% to drastically increase the life cycle of the batteries.
Using the middle 50% or the middle 85% will be about the same. Still going to age over time regardless of use. I expect 10 to 20 years service. The temperature may have a bigger effect. New battery technology may make these obsolete before they are worn out anyway.
 

rmaddy

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I'm trying to set myself up with the 5000w inverter so that I end up with what I want while I can't buy it all at once. The only time I'd be using the 5000w is when I'm cooking and I need to do a quick burst in the microwave (1500w) (15 seconds to 3 1/2 min) while I'm running the heater/ac (1500w) and cooking on the griddle or in the countertop oven (both 1500w) and then my fridge is 345/750(surge). Or if I want to use my oven, ac, and griddle at the same time (4500w at 12v), even if it's for 30 min.
5000W inverter is fine. Just not at 12V. It can be done at 24V but it should only be done at 48V. At 12V you need to wire for 500A. Again, that's absurd. Even if it's only for 30 minutes a day you need the proper setup for it to be safe.

And this is a camper. All electric, unless plugged into shore power, isn't a good plan. You are planning 2 12V 200Ah batteries. That's 5120Wh. Running 5000W will last 1 hour on battery. So if you do all of that stuff for 30 minutes you are using up 50% of your battery already. Since that's all you want to use for some strange reason, you can't use any more battery the rest of the day just to support 30 minutes of cooking and/or A/C.

You really need to reconsider your plans here for it to be viable.
 
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jimcalf

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I agree with @rmaddy. Your plan is unbalanced from the perspective of battery size and desired inverter output. To be honest, your described power requirements even exceed what a 30A shore power connection can support. What size is your current shore power connection? 50A or 30A? Regarding having an oversized inverter output, the Victron Multiplus can output up to 50A 120V AC power, by combining power from the batteries with available shore power. In my setup, I have 6 gauge wire from my Multiplus to my AC power center, so even though I only have a 30A shore power connection (using 10 awg cable) the Multiplus can provide up to 50 amps / 6,000W to my breaker panel. To do that though, you need some big-ish batteries and proper wiring. I run a 24V 460ah battery bank, which supplies the Multiplus nicely for those rare moments when I need the combined power of shore and battery. It's all doable, but balancing the design is important. If you have a generator, and plan to use it to charge your batteries, then buying more battery would be my first move. With the kind of power demand you're describing, 800W of solar might be just a drop in the bucket. Especially if like me, you live up north where clouds and trees are all too common.
 
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