Questions regarding batts and panels

To start; I have 600 watts of solar panels on the roof of the van, an 1800 watt modified sine wave inverter and I believe if I remember correctly a 40 watt charge controller(I may have to correct this later but it's that or 60).

My lovely fiance bought two 200amp hour lifepo4 batteries to replace the lead acids that are dying a horrible death.

My first question is that I know that 600 watts will charge one battery just fine but what about 2?

If it will be near impossible to charge two, how much would be the minimum to boost in panels to cover the 2nd battery?

If it's not enough he's considering some kind of switch where only one battery is used at a time but both can charge at the same time. Anyone ever do this?

He's electrically inclined but solar is new to him. I know just enough to start a fire. Thanks in advance.

Well it depends on how deeply depleted the batteries get. Two fully depleted batteries will take much longer to recharge than one fully depleted battery.

But if you discharge one battery to 50%, or split that between two batteries each drawn down 25%, (leaving 75%) then the amount of time to recharge, (all other factors being equal) will be about the same.

600 watts of solar is plenty for your 400ah of lifepo4. I charge over 500ah of lithium with a 365 watt solar panel. Just make sure to measure the voltage on the battery terminals while charging, they have to reach at least 14.4 - 14.6 volts. If you have voltage drop from controller to the battery, you might not reach 14.4 volts.
If you have voltage drop from controller to battery, the battery will charge too slow.
If you don't have a coulombmeter, get one, you will need it for lifepo4. I use the tk15, it works pretty good for the price (under 50 dollars.)
Make sure both batteries are the same voltage before you connect them together (parallel).


tk15 coulombmeter

Headache said:

if I remember correctly a 40 watt charge controller(I may have to correct this later but it's that or 60).

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So onlookers don't get confused: I imagine this was a typo for 40A or 60A, since those are traditional ratings and line up with 600w of panel.

Headache said:

My first question is that I know that 600 watts will charge one battery just fine but what about 2?

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Depends on depth of discharge, amount of sun, etc. If they are cycled all the way down to 20% SoC (common limit with LFP, like 50% is with lead) it would take 2,048Wh of solar charging power/day to refill. Assuming no other loads.

If we get ~85% yield out of the MPPT after various losses it would require just over 4 hours of Full Sun Equivalent to recharge them from 20% to 100% SoC. See this chart for a crude idea of how much FSE is available at your time/place.

Note that lithium does not have to be charged every day the way lead chemistries do, so with lithium you can "borrow" power from the previous or next day in a way that lead would not appreciate. Another bonus: 600w of panel will charge more like ~660w-690w on litihium (lead's 10-15% charging inefficiency penalty will no longer be present). So I'd let it ride first and see how the situation behaves in actual use.

I would, however, ensure my solar charge controller and any other charging sources have their setpoints configured to match the lithium battery manufacturer's recommendations. The existing controller should be fine if it is configurable.

Headache said:

If it will be near impossible to charge two, how much would be the minimum to boost in panels to cover the 2nd battery?

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I suspect the solar will be sufficient unless you are hammering the bank with big inverter loads regularly. If the same loads/uses are carried over from the lead bank then the Li will probably perform and charge better for the efficiency reasons given above. If the panel rating really is insufficient for your intended time/place then it is just math to see how much more panel would be required. We could help with this. But......

If it's not enough I'd probably add alternator charging to the mix before trying heroic solar measures. Even a $120 20A DC-DC charger can really help solar out if one drives semi-regularly. Counter-argument: I have an isolator for alternator charging my 100Ah Li bank but since I sit in place for 14d whenever possible it doesn't get much of a workout. Every once in a while I relocate on a cloudy day and it earns its keep.

tx2sturgis said:

But if you discharge one battery to 50%, or split that between two batteries each drawn down 25%, (leaving 75%) then the amount of time to recharge, (all other factors being equal) will be about the same.

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Thanks for this. Right now all we have on them 24/7 is the 3.1 cu. ft. fridge. Charging phones and such during the day then a 5 foot LED light strip at night. Oh yes and since it's getting warm, fans. We want to attempt one of my 5000 BTU a/c window shakers.

frater secessus said:

1) So onlookers don't get confused: I imagine this was a typo for 40A or 60A, since those are traditional ratings and line up with 600w of panels.

2) If we get ~85% yield out of the MPPT after various losses it would require just over 4 hours of Full Sun Equivalent to recharge them from 20% to 100% SoC. See this chart for a crude idea of how much FSE is available at your time/place.

3) I would, however, ensure my solar charge controller and any other charging sources have their setpoints configured to match the lithium battery manufacturer's recommendations. The existing controller should be fine if it is configurable.

4)I suspect the solar will be sufficient unless you are hammering the bank with big inverter loads regularly.

5) If it's not enough I'd probably add alternator charging to the mix before trying heroic solar measures...but since I sit in place for 14d whenever possible it doesn't get much of a workout. Every once in a while I relocate on a cloudy day and it earns its keep.

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1) Yes, typos! It's a 40 amp charge controller that automatically detects and resets for lithium. I need a new one because the software for the Bluetooth is obsolete/non-existent so I can't do anything with it right now.

2) I'm urban camping in sunny Riverside county, CA until I get done with radiation treatments. Of course it's cloudy right now.

3) It is configurable and as soon as I find my external cell phone battery charger I'll be gold, but I'd really like to get a new charge controller to cover the 1200-1250 watts in solar panels that will end up on the roof. I'm an energy whore!

4) We won't be hammering it now but both of us are gamers so....

5) I decided not to go the alternator route for a similar reason as you; sitting too long before moving. There's a 72 parking rule that I adhere to, the town knows I'm here and the sheriff's wave at me as they drive by. I'm a good neighbor in an industrial area practically next to my fiance's work. Other than very much disliking van dwelling and cancer treatment life is pretty gold right now.

jonyjoe303 said:

600 watts of solar is plenty for your 400ah of lifepo4. I charge over 500ah of lithium with a 365 watt solar panel. Just make sure to measure the voltage on the battery terminals while charging, they have to reach at least 14.4 - 14.6 volts. If you have voltage drop from controller to the battery, you might not reach 14.4 volts.
If you have voltage drop from controller to battery, the battery will charge too slow.
If you don't have a coulombmeter, get one, you will need it for lifepo4. I use the tk15, it works pretty good for the price (under 50 dollars.)
Make sure both batteries are the same voltage before you connect them together (parallel).

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My fiance read your post and he wants to know what size sampler you are using. When he looked up the TK15 there were 2 different ones in the manual for it. They were 50 amp and 100/350 amp.

Not sure what your budget is like, but there are some upcoming laptops with cpu/gpu combos that are much better for gaming. Not your old 20 pound(feels like) gaming laptops. It might be worth looking into. You can have external monitors, keyboard and mouse. Even with the externals, you'll save on energy consumption and space. With the added benefit of being able to move those items around. Think of a small LCD projector and gaming outside lol
One of the big things would be the heat output of a traditional power supply. I'm close to your area, and more heat isn't needed at all.

for the tk15, the shunt(sampler) required is what your max amp usage will be. Me I never use more then 15 amps, so I have the 50amp one. You said you plan on using a 5000 btu AC, that uses about 600 watts (600watts/13 volts= about 46 amps) . To play it safe the 100 amp shunt should be ok, since when the compresser starts it will probably go over 50 amps.

Headache said:

My first question is that I know that 600 watts will charge one battery just fine but what about 2?......

Click to expand...

Headache said:

Right now all we have on them 24/7 is the 3.1 cu. ft. fridge. Charging phones and such during the day then a 5 foot LED light strip at night. Oh yes and since it's getting warm, fans. We want to attempt one of my 5000 BTU a/c window shakers.

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400Ahr x 12.8V x 80%= 4100 Whr of energy storage. Your 600W of panels will generate the equivalent of ~2000 to 4000 Whr of energy on sunny days, so you have 1-2 days of full sun storage. Sounds like a good arrangement to me. LiFePO4 batteries can take charge as fast as your system can produce it. You'd want a 60A controller, unless you configure your batteries in series for a 24V system, which I'd recommend if you were just starting. 24V reduces wire losses to 1/4 (or you can use smaller wires), and anything associated with the charging and battery outputs needs only half the amp rating. You'd need a buck converter to step down from 24V to your 12V loads (these are cheap), and a new inverter that can take 24V input (not so cheap but the same price as 12V ones).

I think 300-400 Whr per day is pretty typical for a fridge, but the AC will use a lot (600W?) when it's on.... but you should be fine using it a few hrs per day when it's hot.

Headache said:

To start...start a fire. Thanks in advance.

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.
Our photovoltaic:
* six 305-Watt panels, a total of 1,830-Watts.
These float a couple inches above the roof, quieting rain and deflecting direct sun from our ceiling.
They are also a help for birds tap-dancing in the belfry.
.
Our bank:
* six ancient Concorde Lifeline 105ah AGM.
New in 2002, two decades ago.
We started with eight, but one then another decided to retire... so, despite the 'forever' hype, they sometimes get all tuckered out.
Nighty-night, sweet princes of Baja!
.
Nearly two decades full-time live-aboard in our ExpeditionVehicle.
We schedule our electric use for mid-day sun.
With this procedure, we use the sun as our bank, with the AGM as merely a pass-through.
.
One of our brainiac caravan chums calculated our depletion of the physical batteries:
* probably around 3% (three percent)... and this's instantly re-filled by the mid-day sun.
Our electric use includes:
* cooking with one or all three of our induction hot-plates
* heating water with one or both 'sous vide' circulation heaters
* running any of our 750-Watt automotive invertors to charge 20v batteries for hand-held tools and our Black&Decker 20v flashlights.
.
Occasionally, some Esteemed Expert chides us for not 'exercising' our bank, claiming the batteries will croak well-before the warranty expires unless we run them down to half-full.
Sure.
We will keep that in mind.
.
PS:
The warranty expired in 2005, and yet, somehow, our ancient AGM bank keeps trudging along.
.
PPS:
The darling dujour -- LiFePo4 -- has a way to go before we are comfortable with their 'quirks'.
No rush.

LargeMarge said:

The warranty expired in 2005, and yet, somehow, our ancient AGM bank keeps trudging along.

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LargeMarge said:

We schedule our electric use for mid-day sun.
With this procedure, we use the sun as our bank, with the AGM as merely a pass-through.

Click to expand...

Lifeline are exceptional AGM, and 3:1 solar-to-battery ratio is an exceptional setup. Folks who want their lead-chemistry banks to last many years can learn lessons from LM's example.

Too often, we vanfolk tend to buy poorly-suited Pb banks then charge them poorly. And act surprised when they die prematurely.

LargeMarge said:

Occasionally, some Esteemed Expert chides us for not 'exercising' our bank, claiming the batteries will croak well-before the warranty expires unless we run them down to half-full.

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The (self-)Esteemed Experts would do well to review the DoD vs cycle life data. And the manufacturing dates on the batts.

LargeMarge said:

darling dujour -- LiFePo4 -- has a way to go before we are comfortable with their 'quirks'.

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To quote Sowell, "There are no solutions. There are only trade-offs". In this use the Lifelines are getting optimal conditions for Pb: (very) shallow discharge and robust charging. They may outlive us all.

People with less room for panel, charging from alternator alone, or less weight-carrying ability might find LFP compelling. To get similar capabilities today with the array above we could either spend $2,400 on best-quality AGM (414lb) or spend $1,200 for a 206Ah SOK LFP (48lbs). The ~19Ah overnight load (630Ah x .03 ) could be met easily with 100Ah of LFP but the current requirements of the induction, etc, make the 206Ah a better fit.

I use and have had good service from both Pb and Li. I now run a small li bank. The main difference is I can camp in areas like forests with very limited access to direct sunlight. As long as I get 2hrs/day of good sun the bank will get recharged.

Looks like we have to get another charge controller. I'm not finding a way to get it to charge the bats above 14 and I believe 14.6 is where they top off at. It sees the available amp hours but not the max amp capacity.

That being said I went from batteries that had very little amps left to so many ah storage that my PC is definitely getting hooked up for the weekend. Oh Dougmuffin....bigger fridge time!

Need another inverter as well. This time it needs to be pure sine wave. Any recommendations? I need at least 2000 wats but preferably 3k.

frater secessus said:

...[those] with room for panel, charging from alternator alone, or less weight-carrying ability might find LFP compelling. To get similar capabilities today with the array above we could either spend $2,400 on best-quality AGM (414lb) or spend $1,200 for a 206Ah SOK LFP (48lbs)...

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.
Excellent point!
.
Our GVWR -- 29,000#.
Weight across the scale -- 14,000#.
Cargo capacity -- about seven ton.
.
Although the rig roof is maxed with photovoltaic, our toy-hauler is barren.
And at 8x16 instead of the rig's 7.5x12, the toy-hauler could just about go hog-wild with additional photovoltaic panels plus another half-ton of AGM.
[heart flutters in anticipation, wipes tears of joy from glistening cheeks]
.
Maybe someday, we might build a new-fangled bank using some random collection of LiFePo4 cells we scrounged from some dusty warehouse or barn.
We avoid mail-order, the concept just rubs us the wrong way.
Apparently, we are constitutionally wired to get stuff from people we know, local-owned and family-operated... and, apparently, this shall not be infringed.

Headache said:

Looks like we have to get another charge controller. I'm not finding a way to get it to charge the bats above 14 and I believe 14.6 is where they top off at. It sees the available amp hours but not the max amp capacity.

Click to expand...

LiFePO4 charges quite nicely/fully at 14.0v, and somewhat more slowly at lower voltages. 14.6v is generally the "BMS shuts down the party to avoid damage" point. By that time one or more cells has likely run far away.

I charge mine to 13.8v and cells stay nicely balanced around 4mV.

It sees the available amp hours but not the max amp capacity

Click to expand...

This is likely a function of the reset point, the voltage at which the monitor sets reported capacity at 100% if amp counting hadn't gotten there already.

I like to charge my lithium's up to 13.6.

frater secessus said:

LiFePO4 charges quite nicely/fully at 14.0v, and somewhat more slowly at lower voltages...I charge mine to 13.8v and cells stay nicely balanced around 4mV.

Click to expand...

Is your charging that simple? Say just max watts of input (no limit) at <13.8v then hold 13.8v when it's reached?

Headache said:

To start; I have 600 watts of solar panels on the roof of the van, an 1800 watt modified sine wave inverter and I believe if I remember correctly a 40 watt charge controller(I may have to correct this later but it's that or 60).

My lovely fiance bought two 200amp hour lifepo4 batteries to replace the lead acids that are dying a horrible death.

My first question is that I know that 600 watts will charge one battery just fine but what about 2?

If it will be near impossible to charge two, how much would be the minimum to boost in panels to cover the 2nd battery?

If it's not enough he's considering some kind of switch where only one battery is used at a time but both can charge at the same time. Anyone ever do this?

He's electrically inclined but solar is new to him. I know just enough to start a fire. Thanks in advance.

Click to expand...

600 watts of solar panels are never actually 600 watts. usually less by a small amount even under ideal conditions.
Batteries are never exactly the same.
There is a device called a "BMS" that you can buy that will take that difference into consideration so that each battery is charged equally and at the proper rate until full.

Your high priced batteries may have a "BMS" built in. check before going any further or buying switches and such.

Just in case here are some numbers that are simple to understand.
use your 600 watt panels to start. Use the voltage of your batteries, lets say 12 volts.
use your hand held calculator.
divide 12 into 600.. that will result the amps going into your batteries. (under idea conditions)
Another way is the reveres Volts times amps equal power in watts.
Another factor that needs to be considered is watt hours which is just a confusing way of saying how long things have been turned on, Lets say a toaster. On the label it will tell you how many watts it uses while toasting, (about 1000 watts).
But only for a short time. 2 or 3 minutes.
So- You are using .2 to .3 watt hours. lots of toast in a whole hour of toasting. But if you did, that would be one kilowatt hour. 1000 watts for 1 hour.
switches will not be needed.

rruff said:

Is your charging that simple? Say just max watts of input (no limit) at <13.8v then hold 13.8v when it's reached?

Click to expand...

not with lifepo4. Lifepo4 reads 13.1 volts the majority of the time, it won't take long to reach 13.8 volts. My charge controllers wouldn't be charging the battery at more than 1 to 2 amps at 13.8 volts.
I keep my charge controller bulk voltage at about 14.2 volts when the 220ah lifepo4 isnt being used too much and it charges at max about 8 amps and never trips the bms, during extremely hot weather where I use my swampcooler all day long plus fridge, I keep the bulk voltage at 14.4 - 14.6 volts, at those voltages there is a chance to trip the bms, it maxes out at about 16 amps. The larger the separation between the the bulk voltage and the battery voltage the more amps the battery receives.
Thats why the calibration of the charge controller and battery terminals is very important. The charge controller can be reading 13.8 volts but the battery terminals 13.4 volts, The controller will think the battery is fully charge, when its actually undercharge.

wb8vyn said:

600 watts of solar panels are never actually 600 watts. usually less by a small amount even under ideal conditions.
Batteries are never exactly the same.
There is a device called a "BMS" that you can buy that will take that difference into consideration so that each battery is charged equally and at the proper rate until full.

Your high priced batteries may have a "BMS" built in. check before going any further or buying switches and such.

Just in case here are some numbers that are simple to understand.
use your 600 watt panels to start. Use the voltage of your batteries, lets say 12 volts.
use your hand held calculator.
divide 12 into 600.. that will result the amps going into your batteries. (under idea conditions)
Another way is the reveres Volts times amps equal power in watts.
Another factor that needs to be considered is watt hours which is just a confusing way of saying how long things have been turned on, Lets say a toaster. On the label it will tell you how many watts it uses while toasting, (about 1000 watts).
But only for a short time. 2 or 3 minutes.
So- You are using .2 to .3 watt hours. lots of toast in a whole hour of toasting. But if you did, that would be one kilowatt hour. 1000 watts for 1 hour.
switches will not be needed.

Click to expand...

jonyjoe303 said:

not with lifepo4. Lifepo4 reads 13.1 volts the majority of the time, it won't take long to reach 13.8 volts. My charge controllers wouldn't be charging the battery at more than 1 to 2 amps at 13.8 volts.
I keep my charge controller bulk voltage at about 14.2 volts when the 220ah lifepo4 isnt being used too much and it charges at max about 8 amps and never trips the bms, during extremely hot weather where I use my swampcooler all day long plus fridge, I keep the bulk voltage at 14.4 - 14.6 volts, at those voltages there is a chance to trip the bms, it maxes out at about 16 amps. The larger the separation between the the bulk voltage and the battery voltage the more amps the battery receives.
Thats why the calibration of the charge controller and battery terminals is very important. The charge controller can be reading 13.8 volts but the battery terminals 13.4 volts, The controller will think the battery is fully charge, when its actually undercharge.

Click to expand...

Another comment about reading voltages to determine battery condition.
Every test point must use the same measuring devise, Controller display vs hand held meter at battery connection.
There are wires that have an internal resistance, So from one end to the other will be different and the calibration from one measuring device to another will be different.
So---- use one meter at all points with all loads shut off. The differences will be less under those conditions.
Then pick a point in the system (at the batteries). Set the controller to deliver the wanted max voltage under those conditions (at the battery terminals) and then leave it alone. Let the BMS and Controller do the rest and sleep well.