How can I limit charge current for smaller AGM battery (Rover 20, 200W PV)

[Joe del Sol]

Hi all. I'm trying to add a small PV system to my travel trailer for occasional boondocking use (lights and cell phone charging in the evening). The current house battery is an 80Ah AGM. (Upgrade to LiFePO₄ is in the future, but not yet). I understand an AGM charge rate should be from 0.1C (8A) to 0.2C (16A). Rates 0.2C and above are said to shorten battery life.

First iteration was a 100W panel ((Imp): 4.4A, (Vmp): 22.3V) + Renogy Wanderer 10 PWM. We generally camp in forested areas with broken shade (yes, not ideal for solar). In an attempt to capture more energy in those conditions, I ordered a Rover 20 MPPT and another (identical) 100W panel.

My questions:
1) The Rover 20 + 200W PV, under ideal conditions, would send too much current (0.2C) to the house battery. Is there a way to limit charging current to 10A?

2) Seems like an ideal setup (with this battery) would be a 10A MPPT (EPEver makes one) and over panel it (there's been discussion that it won't handle over panelling, but that's another discussion). I'd much rather buy and install one (larger) controller for use when we upgrade to LFP batteries, so I'm really hoping the Rover 20 would work somehow. Return it and get the EPEver instead?

3) After typing all this, just realized maybe the best compromise is wiring the two panels in parallel, using the old 10A PWM and returning the Rover 20. This would give max 8.8A charge to the battery, which is just over 0.1C, but of course, less power than an MPPT.

Thoughts? In short, what's the best way to use lots of PV real estate (because of the shade) while limiting charge current?

 

[DThames]

Hi all. I'm trying to add a small PV system to my travel trailer for occasional boondocking use (lights and cell phone charging in the evening). The current house battery is an 80Ah AGM. (Upgrade to LiFePO₄ is in the future, but not yet). I understand an AGM charge rate should be from 0.1C (8A) to 0.2C (16A). Rates 0.2C and above are said to shorten battery life.

First iteration was a 100W panel ((Imp): 4.4A, (Vmp): 22.3V) + Renogy Wanderer 10 PWM. We generally camp in forested areas with broken shade (yes, not ideal for solar). In an attempt to capture more energy in those conditions, I ordered a Rover 20 MPPT and another (identical) 100W panel.

My questions:
1) The Rover 20 + 200W PV, under ideal conditions, would send too much current (0.2C) to the house battery. Is there a way to limit charging current to 10A?

2) Seems like an ideal setup (with this battery) would be a 10A MPPT (EPEver makes one) and over panel it (there's been discussion that it won't handle over panelling, but that's another discussion). I'd much rather buy and install one (larger) controller for use when we upgrade to LFP batteries, so I'm really hoping the Rover 20 would work somehow. Return it and get the EPEver instead?

3) After typing all this, just realized maybe the best compromise is wiring the two panels in parallel, using the old 10A PWM and returning the Rover 20. This would give max 8.8A charge to the battery, which is just over 0.1C, but of course, less power than an MPPT.

Thoughts? In short, what's the best way to use lots of PV real estate (because of the shade) while limiting charge current?

If you look closely at the EPEver manual (AN series) you will see that it current limits itself at the rated current.

 

[MisterSandals]

1) The Rover 20 + 200W PV, under ideal conditions, would send too much current (0.2C) to the house battery. Is there a way to limit charging current to 10A?

200W / 14V charging = 14.28A if operating at 100% efficiency

If you get an SCC that allows setting the charge current that would solve most of your issues. A Victron 100/30 would be an excellent investment.

 

[rmaddy]

Some (most?) SCCs can be configured to a user specified max charge current. Check to see if your planned SCC can be configured this way.

For example, I have a Victron 150/45 which has a max charge current of 45A. My battery prefers a 40A charge current so I set the SCC to a max of 40A.

 

[Joe del Sol]

200W / 14V charging = 14.28A if operating at 100% efficiency

If you get an SCC that allows setting the charge current that would solve most of your issues. A Victron 100/30 would be an excellent investment.

Thanks. Good point on the 14.28A. I didn't know if the charging rate recommendations were based on charging voltage or nominal battery voltage (so, I went conservative). 14.28A would be a charge rate of ~0.18C, which I'd consider acceptable.

Victron hardware will definitely be used if/when we decide to get serious about boondocking. Right now, we're in the testing phase, thus using less expensive components (and the existing battery and converter). I'll consider a 100/30 or 100/50 if my Rover 20 won't limit charging current.

 

[12VoltInstalls]

hoping the Rover 20 would work somehow. Return it and get the EPEver instead?

Yes, return imho

sing the old 10A PWM and returning the Rover

yes

thus using less expensive components (and the existing battery and converter).

for $425 the MPP 1012LV:
1) has setable charging output
2) eliminates the need for the coinverter
3) provides shorepower charging should that be desired
4) has 1000W inverter built in
5) has automatic transfer switch built in
6) has (iirc) 500W panel capacity
7) has mode for pass-through of shorepower when/if batteries are low
8) can wire direct to camper fusel box.
9) can be programmed for agm, flooded, or agm

I think that would work so good for a small camper you won’t look back

 

[Joe del Sol]

Some (most?) SCCs can be configured to a user specified max charge current. Check to see if your planned SCC can be configured this way.

For example, I have a Victron 150/45 which has a max charge current of 45A. My battery prefers a 40A charge current so I set the SCC to a max of 40A.

The Rover 20 manual (https://www.renogy.com/content/RNG-CTRL-RVR40/RVR203040-Manual.pdf) states "Battery charging parameters can also be programmed using the Renogy BT APP. " BT APP manual: https://www.renogy.com/template/files/RNG-180206-BT-1 APP.pdf


I'm using the app now, and see only voltages and times that can be changed, no output current limiting. Ah well, looks like it's time to buy Victron and be done with it. In the meantime, I'm going to parallel the two panels already installed to the PWM controller, which should get us by for awhile.

 

[Joe del Sol]

for $425 the MPP 1012LV:
1) has ...

I think that would work so good for a small camper you won’t look back

Thanks for the suggestion. I'll look into how that could tie into our existing on board converter (WFCO 8955-AN).

The usual suggested upgrade path is to replace the charger board with a Progressive Dynamics PD4655LIV to make it lithium compatible (14.4V charging, no float). However, with the greater charging capacity (55A) from the new board, the 10ga factory wiring needs to be replaced. One of many reasons I'm still trying to get by with the existing SLA battery.

 

[12VoltInstalls]

no output current limiting

Exactly

I'll look into how that could tie into our existing on board converter (WFCO 8955-AN).

you don’t need the converter with this and many AIOs. You could even sell it away.
My camper doesn’t have a converter and you don’t need it with an AIO
You don’t want them tied in..

usual suggested upgrade path is to replace the charger board with a Progressive Dynamics PD4655LIV to make it lithium compatible (14.4V charging, no float).

Not here!

The/a converter does a job for the unthinking masses.

It doesn’t do the job well nor are they tailorable to specific situations.

If an AIO that charges batteries well is opted, there is no need for the poor function converter.
The AIO can be programmed for flooded lead acid or lithium or agm.

Present and future problems solved.

 

[Substrate]

Huh, wait. 80ah * .20 = 16a continuous charge possible. Most generic agm's can actually handle 0.25C efficiently. (pure leads different story)

200w panel / 14v charging is wrong. The canonical middle-of-the-road value is to divide by 18

200w panels / 18v = 11.1a charging under absolute best conditions. Knock some real world off that for 10A by napkin calc. Youve got 6A available.

A 300w system (300w / 18 = 16.6A - fine enough for 0.2 and even more if 0.25 is considered.

The whole point though with AGM is take advantage of it's ability to go to 0.2 - 0.25C so you can charge as fast as possible with limited sun.

If you don't take advantage of agm's ability to charge at faster rates, like 0.2 / 0.25C, then you run the risk of under-charge, especially if you are "daily cyclic", which is far more damaging than pushing the current from your panel system to the max 0.25C.

I know, application-specific propeller head material, but its part of the reasons 99.9% of agm's die an early death.

 

[Joe del Sol]

200w panel / 14v charging is wrong. The canonical middle-of-the-road value is to divide by 18
...

The whole point though with AGM is take advantage of it's ability to go to 0.2 - 0.25C so you can charge as fast as possible with limited sun.

If you don't take advantage of agm's ability to charge at faster rates, like 0.2 / 0.25C, then you run the risk of under-charge, especially if you are "daily cyclic", which is far more damaging than pushing the current from your panel system to the max 0.25C.

I know, application-specific propeller head material, but its part of the reasons 99.9% of agm's die an early death.

Thanks for all that. Just wondering - why use 18V for the amp charge calculation? I'm not disputing it, just want to use the right values for future calculations. (Resistance?)


Interesting note on the higher charge rates and "under-charging" leading to a shorter life. I've always tried to baby my SLA and flooded batteries by charging slowly and keeping them topped off.


...still learning here.

 

[12VoltInstalls]

why use 18V for the amp charge calculation? I'm not disputing it, just want to use the right values for future calculations. (Resistance?)

The max (ideal sun conditions at ~76*F) panel output under load (connected to an SCC that is charging) is around ~18V.

 

[MisterSandals]

200w panel / 14v charging is wrong. The canonical middle-of-the-road value is to divide by 18

What?
He has a 200W panel and is charging is battery at roughly 14V.
400W / 14V = 28.5A charging (at 100% efficiency)

Thanks for all that. Just wondering - why use 18V for the amp charge calculation? I'm not disputing it, just want to use the right values for future calculations.

I am wondering too. I suspect he thinks I was using panel voltage.
But maybe I need to understand “canonical middle-of-the-road” better.