Amp Hours--I don't get it

[DanDweller]

I want to point out that the larger 170ah battery will provide more usable power, BUT...if it is depleted after a few days of heavy usage with little or no solar, then it may be a bit harder to bring it up to full charge (with 200 watts of solar and high power fan operation, lights, fridge, and other items).

Attaining full charge (at least occasionally) is crucial to LFP batteries because that is when 'top balancing' occurs.

If the OP depletes that 170ah battery because there seems to be a 'bottomless well' of power, then it may be difficult to bring it back to full charge with only 200 watts of solar, especially during a week of cloudy weather. If those panels are portable then they can be moved to track the sun and will perform much better, but if they are roof mounted and fixed in place, the overall daily output is going to be less.

200 watts of solar will make around 10-12 amps under full sun, and less when angled, shaded, hot, or on cloudy days, and a depleted 170 ah LFP will consume all of it, but it will take about 17-20 hours (maybe more) of full output from the panels to fully recharge and balance the cells.

For the $500 price difference, I would buy the 100ah LFP and then buy another solar panel or two.

Not a deal killer but something to consider.

Thanks, good point.  Yeah, I'll have the solar panels but also have it set up to be able to charge using the alternator, or shore power if need be.  Never had panels in fact--I just don't want to have to rely so heavily on running the engine or connecting to shore power like I've always done.  This makes me wonder: How long should it take to recharge a depleted 170Ah battery using the alternator?  Don't the lithium batteries charge quicker than their lead-acid counterparts?  Also, I need to get clear on how to know when the battery is fully charged.  Is there a display or indicator on the solar charge controllers?  Also I don't really understand how to keep from overcharging it (like I once did and fried a lead-acid battery by leaving it plugged into shore power too long).  Am I correct in understanding that the lithium batteries have a built-in mechanism to avoid overcharging?  Is that too many questions?

 

[DanDweller]

For the $500 price difference, I would buy the 100ah LFP and then buy another solar panel or two.

Not a deal killer but something to consider.

Oh I only just now noticed what you said here.  So instead of investing in the larger battery, maybe better to get the smaller one and instead invest in enough panels to keep up.  I still need to measure my roof and see how many watts' worth of panels I can fit on it (it's a Ford Transit Connect, long wheel base).  For that matter, I need to get to the bottom of whether it is worth messing with the flex panels (which I would prefer ideally for aerodynamics and to avoid standing out so much).

 

[tx2sturgis]

This makes me wonder: How long should it take to recharge a depleted 170Ah battery using the alternator?  Don't the lithium batteries charge quicker than their lead-acid counterparts?  Also, I need to get clear on how to know when the battery is fully charged.  Is there a display or indicator on the solar charge controllers?  Also I don't really understand how to keep from overcharging it (like I once did and fried a lead-acid battery by leaving it plugged into shore power too long).  Am I correct in understanding that the lithium batteries have a built-in mechanism to avoid overcharging?  

This is complicated, but essentially, the alternator on a Transit Connect (or any smaller car) might not be up to the task of fully charging a depleted LFP (LiFePo4) battery. Or, the alternator might get overloaded. LFP batteries can pull a LOT of current when recharging. You will need a way to limit the drain on the alternator. And you will need a way to bring the LFP battery up to 14.2-14.6 volts and hold it there for a while, which your alternator probably can't do.

You should NOT simply parallel an LFP battery with your vehicle lead-acid starting battery thru a switch or solenoid.

A battery-to-battery charger with LiFePo4 compatibility is one option. Renogy makes one:

https://www.renogy.com/renogy-12v-dc-to-dc-on-board-battery-charger/

Also, the solar charge controller you will be using for the LFP battery will (or should) have the proper charge profile for charging the LFP. Once the battery is fully charged, the built-in BMS in the battery will begin the cell balancing process. As this is happening, the voltage will show about 14.2-14.6 or so, and the solar input current will drop, then taper off to almost nothing, and stay there. At this point the controller stops charging and the LFP battery can be considered fully charged.

 

[tx2sturgis]

So instead of investing in the larger battery, maybe better to get the smaller one and instead invest in enough panels to keep up.  

That's what I would do, if I only had room for 200 watts on the roof, then an additional 100 watt portable panel can be used to boost the power input when needed. These panels will all need to be wired in parallel configuration if using one solar controller, PWM type. 

You could use two controllers, one of the MPPT type for the 200 watt roof array (in series) and then a simple PWM controller for the portable panel. Both of these need to be LFP compatible controllers. There are some expensive controllers that can be 'networked' in dual operation to work in tandem....this is not the way I would do it but I mention it because if I dont, someone surely will. For a 200-300 watt system with one battery, running a fridge, a fan, and a few lights, I don't see this option as cost-effective.

 

[tx2sturgis]

I found a good explanation of how you can charge LFP batteries with a vehicle alternator.

https://shop.pkys.com/Alternator-Lithium-Battery.html

This website is explaining a marine system, but the technology used is similar to what would be needed if you REALLY need to charge a lithium battery bank with your alternator.

Actually, the Renogy solution I posted earlier is a LOT less complicated.

For the combination system I built, I avoided charging the LiFePo4 battery from the vehicle. It is charged primarily with solar, which works very well, with a shore power charger that can interface with the solar/battery/inverter on my pickup, OR can be plugged into shore power directly.

Keeping the alternator/lead-acid system completely separate from the solar/LFP system was the easiest, most reliable, and cheapest method for me.

 

[DanDweller]

I found a good explanation of how you can charge LFP batteries with a vehicle alternator.

https://shop.pkys.com/Alternator-Lithium-Battery.html

This website is explaining a marine system, but the technology used is similar to what would be needed if you REALLY need to charge a lithium battery bank with your alternator.

Actually, the Renogy solution I posted earlier is a LOT less complicated.

For the combination system I built, I avoided charging the LiFePo4 battery from the vehicle. It is charged primarily with solar, which works very well, with a shore power charger that can interface with the solar/battery/inverter on my pickup, OR can be plugged into shore power directly.

Keeping the alternator/lead-acid system completely separate from the solar/LFP system was the easiest, most reliable, and cheapest method for me.

Thank you very much for all the information.  This is very helpful.  I had been wondering how to go about using the alternator to charge the secondary battery and had doubts about its ability to do so, but was saving those questions for down the road--but now I guess I have a game plan and will probably get the Renogy device you showed me.  One thing I also wonder is whether I could set things up so that if my drive battery is dead I could draw from the lithium battery to start the van--or perhaps the lithium battery isn't suited for cranking amps?

Also, would you mind elaborating on this?:
".... a shore power charger that can interface with the solar/battery/inverter on my pickup, OR can be plugged into shore power directly."  What kind of charger do you have?

Finally, what solar panels are you using and how long have you had them?  I guess this  should probably be another post, but I'm undecided on whether to risk getting flex panels and have them go bad within a year or two vs. getting traditional panels and having more complicated mounting and wind drag.  Thanks!

 

[tx2sturgis]

You can jump start a vehicle with a large LFP battery, in fact you can buy those small lithium 'jump starter' packs to do just that. 

This is not something you want to do all the time, but if you parallel the LFP using jumper cables, it should do no damage to the battery if done carefully. (let the LFP battery charge the starter battery first) They are not really 'starter' batteries but they can deliver a large amount of current if needed. In a typical RV with a generator, they can be used for starting the generator. But of course the current required is much lower than with a vehicle engine.

For shore charging my BattleBorn 100ah battery, I use a Bioenno 10 Amp Lithium compatible charger. I installed anderson connectors to use it when and if needed, which is rare, since the 100 watt portable panel I use works fine. But I have the charger and can plug it in if I need to. They also make chargers with more than 10A capacity.

https://www.bioennopower.com/collec...-anderson-for-12v-lifepo4-batteries-bpc-1510a

 

[maki2]

  Ok, so with the Engel fridge on the hottest day the compressor might be on 80% of the time, so I would be looking at: 2.5*(.8*24hrs)=48Ah.

That figure is not correct because it would only be on 80% of the time during the very hottest hours of the day at 95 degree temperature. It won't be on 80% of the time during the late evening, the middle of the night and the early morning.  Of course if you are not parked in the shade during 95 degree weather and your rig is closed up bringing them temperature above the ambient then it will likely be running at 100% for that time period.
Remember you can add extra insulation around the unit as long as you don't block the vents on the unit. That will help during hot weather. Put some reflective foam insulation with the shiny side facing to the outside. Extra insulation in the summer is worth the extra time to do it. The less you open the fridge the lower your power useage will be.

 

[DanDweller]

For shore charging my BattleBorn 100ah battery, I use a Bioenno 10 Amp Lithium compatible charger. I installed anderson connectors to use it when and if needed, which is rare, since the 100 watt portable panel I use works fine. But I have the charger and can plug it in if I need to. They also make chargers with more than 10A capacity.

So am I right in guessing you don't need it to be a "smart" charger because the battery's built-in management system keeps you from overcharging it?  So, for example, if you went off and left the charger on for too long it wouldn't be a problem?

 

[DanDweller]

That figure is not correct because it would only be on 80% of the time during the very hottest hours of the day at 95 degree temperature. It won't be on 80% of the time during the late evening, the middle of the night and the early morning.  Of course if you are not parked in the shade during 95 degree weather and your rig is closed up bringing them temperature above the ambient then it will likely be running at 100% for that time period.
Remember you can add extra insulation around the unit as long as you don't block the vents on the unit. That will help during hot weather. Put some reflective foam insulation with the shiny side facing to the outside. Extra insulation in the summer is worth the extra time to do it. The less you open the fridge the lower your power useage will be.

Thanks, good to know.  I'll definitely extra-insulate whichever fridge I end up getting.

 

[tx2sturgis]

So am I right in guessing you don't need it to be a "smart" charger because the battery's built-in management system keeps you from overcharging it?  So, for example, if you went off and left the charger on for too long it wouldn't be a problem?

Short answer: No it's not a problem. 

Long answer:

Most portable lithium compatible shore power chargers are....I dunno....lets say 'half-smart'.

Since the built-in BMS inside the battery has some 'smart' in it already, there is no need to duplicate that functionality in the external LiFePo4 battery charger.

All the LFP charger needs to do is charge at full power (called constant-current/constant-voltage or CC/CV) up to the point that cut-off voltage is reached, about 14.4v-14.6v or thereabouts, then it will shut itself off (no more charging occurs). 

So they are either charging, or they are not. There are no 'stages' other than: charging, or not charging. 

Lithium compatible solar charge controllers actually do 'float' the LiFePo4 battery by replenishing the battery as loads are applied, but most portable shore power LFP chargers don't normally do this.

But...'on-board' style (not portable) lithium battery maintainers add a 'maintenance' mode.

These on-board or built-in chargers DO monitor the lithium battery and resume charging when needed. They are 'spendy' and not needed by most van dwellers/boondockers unless they plan to remain plugged into shore power (without solar) for many days or weeks at a time.

Progressive Dynamics has them:

https://www.progressivedyn.com/lithium-battery-power-centers/

I believe I have seen articles on this forum referencing 'combination' shore power/solar lithium chargers but I have no experience with these, maybe others will offer information or ideas.