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Old 12-10-2020, 09:58 AM   #1
Skoolie
 
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Best Solar Backup System

Hey all, I have been living in my short bus for a couple of months now. I am currently in Colorado for the winter. I have 600W of solar and a 414Ah 12v LIFEPO4 battery bank.

So far the solar has been keeping the battery bank pretty well topped off. However, with the days getting shorter I am spending more time inside on my computer as well as getting less sunlight for the solar panels. I would like to have a back up system that I can use to charge the batteries if the solar can't keep up with my needs.

I would like to do this a economically as possible because first of all I am poor (sad day) but also because I will probably only use this back up system once or twice a month during cloudy weeks.

I think my two options are battery to battery chargers from the inverter to the battery bank and a 120v to 12v battery charger. Do you guys have any suggestions of products for either of these types of chargers. I was thinking that as long as I am just in the bulk charging phase of the charging process I would not need to get a specific LIFEPO4 battery charger. Instead I would just need a charger that can get to 14.6V and then I would only charge the batteries to 85 percent or so. Is this conclusion correct or am I missing something?

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Old 12-10-2020, 01:48 PM   #2
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Inverter only uses energy, has to come from

your alt while driving

shore power

and/or inverter genset best when staying off-grid.

Same charger for the last two, sized for 80% of the genset output.

The spec'd 14.6V is way too high for optimal longevity, 13.8V much better, still over 90%

But you need access to the cell voltages and understand how the BMS does balancing.
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Old 12-11-2020, 08:42 PM   #3
Skoolie
 
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Quote:
Originally Posted by john61ct View Post
Inverter only uses energy, has to come from

your alt while driving

shore power

and/or inverter genset best when staying off-grid.

Same charger for the last two, sized for 80% of the genset output.

The spec'd 14.6V is way too high for optimal longevity, 13.8V much better, still over 90%

But you need access to the cell voltages and understand how the BMS does balancing.

Yeah, for sure the B2B battery would only be charging while driving. Worst case scenario I should be able to idle when the batteries are real low.

I already have an inverter so Id probably just be getting a stand alone charging unit. And like I said I won't be using much.

I was talking about charging voltage at 14.6V not charging the batteries up to 14.6V. I have my float voltage at 13.9V for my current battery bank. My question is if I am only charging from lets say 20% to 70%, so just bulk charging, could I get away with just a generic charger since I won't be doing float voltage or anything like that to top the batteries off. I will rely on solar for that.
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Old 12-11-2020, 09:02 PM   #4
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Originally Posted by mitchk View Post
I was talking about charging voltage at 14.6V not charging the batteries up to 14.6V
So was I, that is way too high for optimal longevity.

Actual 100% for LFP is 3.33-3.35Vpc

If your charge is low C-rate like solar a CV setpoint (maximum charge voltage) of 3.38V will get you there.

At a high C-rate you could use 3.5V, but do not hold CV / Absorb stage too long

In reality no CV / Absorb stage at all is healthier, just "charge **to** the setpoint" and stop.

Floating is for lead, LFP should **not be floated at all**, no matter how low the voltage.

Even just sitting isolated at 100% is not ideal for more than an hour or two

when not actively feeding loads, SoC should be down below 60%, lower the better, so long as there is no risk of dropping below 3Vpc, completely isolate the bank until it will be needed, charge to higher SoC as close to that time as possible.

TLR Lead needs to sit at 100% for as much of its lifespan as possible, but that is harmful to LFP longevity.
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Old 12-11-2020, 09:21 PM   #5
Skoolie
 
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Originally Posted by john61ct View Post
So was I, that is way too high for optimal longevity.

Actual 100% for LFP is 3.33-3.35Vpc

If your charge is low C-rate like solar a CV setpoint (maximum charge voltage) of 3.38V will get you there.

At a high C-rate you could use 3.5V, but do not hold CV / Absorb stage too long

In reality no CV / Absorb stage at all is healthier, just "charge **to** the setpoint" and stop.

Floating is for lead, LFP should **not be floated at all**, no matter how low the voltage.

Even just sitting isolated at 100% is not ideal for more than an hour or two

when not actively feeding loads, SoC should be down below 60%, lower the better, so long as there is no risk of dropping below 3Vpc, completely isolate the bank until it will be needed, charge to higher SoC as close to that time as possible.

TLR Lead needs to sit at 100% for as much of its lifespan as possible, but that is harmful to LFP longevity.
Gotcha. For some reason I was thinking that during bulk charging with my victron charge controller it was charging at around 14.6V but I must be mistaken. My charge controller has a preset for LiFePo4 batteries but I lowered the absorption voltage from 14.2 to 13.9. The absorption phase is 2 hours. The float voltage is set as 13.5. I don't see a way to disable the float. Do you think I should change these settings?
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Old 12-11-2020, 09:28 PM   #6
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I have hardly **ever** seen a canned LFP preset that was not way too high

everyone assumes the mfg data sheet for "maximum voltage" should be what you use for normal cycling charge setpoint

it's not.

it is a very stressful "do not normally approach" boundary point, past which "immediate damage" can occur.

Suitable only for high C-rate use cases like racing, where every mAh you can squeeze in matters, and longevity is not at all a concern.

The key for LFP charge sources is **not** a "lithium" sticker, but user-custom adjustability of the setpoints.
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Old 12-11-2020, 09:35 PM   #7
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Originally Posted by mitchk View Post
Do you think I should change these settings?
Whatever you can do to achieve the desired result. 13.9V is higher than I'd use for CV setpoint!

Realize you may want to keep the SC output going, I'm just saying isolate your LFP bank from that circuit.

An adjustable HVC device would do that. Ideally based on cell-level as a BMS does for fail-safe protection, if the bank is out of balance you want the first / highest / weakest cell hitting your 3.45V setpoint to take the bank offline, a 13.8V HVC at the pack level does not do that.

Does your BMS have any comms protocol, or at least display the per-cell/group level voltages?
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Old 12-11-2020, 09:36 PM   #8
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The canned settings for GEL may be better?
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Old 12-11-2020, 09:54 PM   #9
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Originally Posted by john61ct View Post
Whatever you can do to achieve the desired result. 13.9V is higher than I'd use for CV setpoint!

Realize you may want to keep the SC output going, I'm just saying isolate your LFP bank from that circuit.

An adjustable HVC device would do that. Ideally based on cell-level as a BMS does for fail-safe protection, if the bank is out of balance you want the first / highest / weakest cell hitting your 3.45V setpoint to take the bank offline, a 13.8V HVC at the pack level does not do that.

Does your BMS have any comms protocol, or at least display the per-cell/group level voltages?
I clearly have a ton to learn because I am sort of confused at this point. I am using 3 valence u27-12xp in parallel. I thought using 13.9V absorption was on the safer end but I guess if 3.45 is the desired high point for a cell 13.8V might be a better option? 3.65V is generally the max cell voltage right? But its best to stay way clear of that.

I have the Valance program that I use to try and balance the cells for each battery. Unfortunately I don't believe it shows the individual cell voltage. From my understanding it does a top level charge and uses a resistor to decrease high cell voltages and level them all out.

I am still very new to all of this stuff and trying to wrap my head around it all.
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Old 12-12-2020, 12:23 AM   #10
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The Valance products have a great rep, known-good cell quality, if purchased new will likely last 10+ years even without "optimal" care.

Not familiar with their electronics sorry, but do over time see if you can find a way to get to cell/group level voltages.

Between 13.8 and 13.9V at 4S is a difference of 0.25Voc, I'd say don't sweat it,

just stay far away from 14.6V, afaik the Victron SCs are user-custom adjustable.

The "ideal care" notes I gave you may not be practical to fully automate using commercial OTS gear, many do not want to fuss too much, just do what you can,

so long as no "major events" are allowed to occur (fast charging in sub zero temps, high or low voltage way out of range ) you should see many thousands of cycles.
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Old 12-12-2020, 09:29 AM   #11
Skoolie
 
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Originally Posted by john61ct View Post
The Valance products have a great rep, known-good cell quality, if purchased new will likely last 10+ years even without "optimal" care.

Not familiar with their electronics sorry, but do over time see if you can find a way to get to cell/group level voltages.

Between 13.8 and 13.9V at 4S is a difference of 0.25Voc, I'd say don't sweat it,

just stay far away from 14.6V, afaik the Victron SCs are user-custom adjustable.

The "ideal care" notes I gave you may not be practical to fully automate using commercial OTS gear, many do not want to fuss too much, just do what you can,

so long as no "major events" are allowed to occur (fast charging in sub zero temps, high or low voltage way out of range ) you should see many thousands of cycles.
Well that gives me some hope that my battery bank will last for a good long time. I am being careful not to let any major events happen. I want to try and get a temperature sensor for my victron charge controller to monitor the temperature of the batteries to prevent charging in low temps. So far I have just been checking manually.

One of the things I was possibly thinking about incorrectly is the absorption voltage. This voltage is the just the set voltage being sent to the battery, not necessarily the voltage the battery will be at at the end of the phase. Is that correct?

I lowered my float voltage down to 13.35 because I was told by a few valence owners that battery voltage is around 95 percent charged. Id rather that then going up to 100 percent every summer day.
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Old 12-12-2020, 10:00 AM   #12
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I have a similar setup as you.

I have a Renogy Inverter-Charger to help me recharge from shore power if I have to. I have not used it to give a recommendation and give Renogy’s exceedingly poor customer service I will just leave it at that.

I’ve been thinking about a generator. At first I was thinking of mounting on the bumper, but I found that the little 2KW Honda generator will fit in a hatch I installed in the side under deck and it would be big enough to run a mini split if I decide To do that. Harbor freight and some other resellers have little 1KW generators for really cheap that may be the way to go if you just want to charge your bank
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Old 12-12-2020, 02:20 PM   #13
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Originally Posted by mitchk View Post
Well that gives me some hope that my battery bank will last for a good long time. I am being careful not to let any major events happen. I want to try and get a temperature sensor for my victron charge controller to monitor the temperature of the batteries to prevent charging in low temps. So far I have just been checking manually.



One of the things I was possibly thinking about incorrectly is the absorption voltage. This voltage is the just the set voltage being sent to the battery, not necessarily the voltage the battery will be at at the end of the phase. Is that correct?



I lowered my float voltage down to 13.35 because I was told by a few valence owners that battery voltage is around 95 percent charged. Id rather that then going up to 100 percent every summer day.
Yes LFP has no need to even get that full, sacrificing the use if those top % caoacity is always healthy.

The CV / Absorb charge setpoint is just one factor that determines the "resting isolated" battery voltage result.

The current (C-rate) and AHT (hold time at CV stage) are the other two

all measured at the bank posts.

The higher the current and shorter the time, the greater the voltage will settle.

A complete profile spec , e.g.

Charge to 13.6V and hold CV until amps trail down to 0.05C

as opposed to

Charge to 13.9V at 0.4C and stop, no CV stage

may lead to exactly the same resting voltage and SoC% (capacity usage)

That finish result is what is key, there are a hundred slightly varying ways to get there.

If the resting result is higher than 3.33-3.35Vpc that is just "surface charge" and a sign you stressed the bank went higher / longer than needed
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Old 12-12-2020, 08:21 PM   #14
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I need to review LFP charging/discharging protocols also. My conditioned brain wants to treat my LFP like deep cycle lead acid batteries but I’m adjusting slowly. Ive just been testing these LFP in my bus for a couple weeks but have been too busy with other things to really understand them. But now I made it safely to AZ I can focus and try to adjust this Prostar Mppt 40m charge controller properly. The first time failed. I have it in the “custom” mode so I thought I can adjust all the settings. But it still went into “absorb” and “float” stage. All I want is “bulk” stage charging then for it to stop charging all together at the CV setting. Morningstar has a white paper on LFP charging that is just plain confusing. It doesn’t say what mode to start in. It says to set “absorption” to 14.00 BUT then set the high voltage disconnect to 13.88 so it is always in “bulk” and never goes into “absorption” stage. However the owners manual calls it “Load High Voltage Disconnect”. Does it only disconnect the load (I’m using the Prostar as my load control, LVD, also) or does it disconnect charging also? (I’m thinking it disconnects charging to protect loads sensitive to high voltage?). Does this make sense?
I understand to set CV to 13.8. But I’m not clear what voltage to set the low voltage disconnect. How low can I cycle these? My addled brain keeps wanting to treat them like Lead acid and not cycle them to a low voltage range. Ive seen these LFP dip to 12.2 under load of the water heater then they come back up about 0.75 volts after the heater shuts off. I measured 75 amps (12 volt) with the water heater on. Is that too much for this little 200 ah LFP? These are CALB cells. They have been staying balanced to 0.01 volts difference so far.
I hope this helps the OP.
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Old 12-13-2020, 05:11 PM   #15
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it still went into “absorb” and “float” stage. All I want is “bulk” stage charging then for it to stop charging all together at the CV setting
Many sources cannot, have a minimum AHT, say 10min would be fine


> “Load High Voltage Disconnect”. Does it only disconnect the load (I’m using the Prostar as my load control, LVD, also) or does it disconnect charging also? (I’m thinking it disconnects charging to protect loads sensitive to high voltage?). Does this make sense?

Well it may just stop output to the loads in order to let charging go higher.

Should be able to observe it in action.


> what voltage to set the low voltage disconnect.

3.0Vpc or 12V

if your C-rate is very high (over 0.5C) maybe a little lower.

a bit higher if discharge under 0.1C

Goal should be bounce back to well over 3.0V after resting isolated


> not cycle them to a low voltage range

true they will get (much) longer lifespan with higher average DoD%

But 3-5000 cycles is lots for most, you going to buy higher capacity in order to shoot for 10,000?

> Is that too much for this little 200 ah LFP? These are CALB cells.

Nope top cells they can take it

> They have been staying balanced to 0.01 volts difference so far.

Good sign, don't bother balancing until you see 0.015V deltas persist

Stay a bit further from the shoulders to prevent that occuring.
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Old 12-13-2020, 07:45 PM   #16
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Thanks John61ct.
I think the LVD has a time delay to prevent nuisance tripping at 3.00V. I’ll read the manual again. It might be one minute delay if memory serves?. My biggest load is the 1200 watt water heater. My kettle, induction plate, and Instapot pressure cooker each draw about 800 watts. As long as I only run one at a time it’s been working good enough for this budget bus conversion. I’m going to follow the sun for solar. But the Honda Eu2200i is here just in case. I heat up the water in the middle of the day using solar then turn it off. And the Kisae DC-DC Charger has worked well for me. As I drive this little bus nearly everyday it works well to keep the battery up. A DC-DC charger might not make sense for a big rig that’s parked a lot.
I really like this energy system for how much I have invested. But it still cost me more than the bus did. I’d prefer a slightly bigger LFP battery bank than 4 of these 200ah cells. 280ah cells would fill the space available perfectly.
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Old 12-13-2020, 09:15 PM   #17
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Originally Posted by Doktari View Post
Thanks John61ct.
I’d prefer a slightly bigger LFP battery bank than 4 of these 200ah cells. 280ah cells would fill the space available perfectly.
So, is this a 200ah 12 volt bank? Just trying to figure out how many amp-hours of Lithium I would need.
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Old 12-14-2020, 04:23 PM   #18
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Yes, my house battery is 200ah at 12V. I have a mini bus (one ton cutaway van) and there’s not much space for a bigger battery. For a full size bus I’d definitely suggest a 400-600ah battery. Some folks use a 800ah battery. But you need a powerful way to charge that much battery.
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Old 12-14-2020, 04:43 PM   #19
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Thanks.

From what I see buying LFP 12 volt batteries to get a 600 amp-hour bank is quite expensive, like more than an entire bus.

Though I wouldn't want to buy lead, especially the flooded lead acid variety.

I already have 400 amp-hours at 12 volt from 4 FLA golf cart batteries, which at there peak would be equivalent to a 200 amp-hour LFP in my estimation. A 600 amp-hour bank in lead at LFP efficiencies would be 12 of my FLA GC batteries.

That would be a lot of acid and lead to truck around.
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Old 12-14-2020, 11:48 PM   #20
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From what I see buying LFP 12 volt batteries to get a 600 amp-hour bank is quite expensive, like more than an entire bus.
You do not want to parallel a bunch of 12V units to get up to high Ah capacity, two or maybe three at most.

You get individual cells at 3.2V, at a high enough capacity so you don't need more than a couple in parallel. So 280Ah cells or higher for a 560+Ah bank.

These cells first get paralleled (into "groups") at the 3.2V level

then these groups get serially connected to get to the desired voltage, 4S for LFP 12V.

Same with lead, buy 6V units better than 12V, but best of all 2V cells, wired as above.

Yes LFP can be 6-10x more expensive per usable Ah, but if you care for the properly might last for decades.

Lead in NA markets can be $1/Ah but twice the weight and space.

Nothing in between I'm afraid.
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