# Sizing Charge Controller



## MoonRiver (Sep 2, 2007)

I have 2 300w 24v solar panels and 1 12v lithium battery (100 ah) and will add a 2nd one soon. My calculation for charge controller is:

600w/12v + 25% = 50a + .25(50a) = 62.5a.

Does this mean I need a 70a MPPT charge controller? It seems strange because controllers that meet amp requirements also support much higher input voltage than I need - for example a controller of 150v/70a.


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## Bearfootfarm (Jul 13, 2006)

Read the information here.

https://www.solarpaneltalk.com/foru...-systems/13389-charge-controllers-pwm-or-mppt

https://www.solarpaneltalk.com/forum/off-grid-solar/off-grid-solar-panel-systems


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## MoonRiver (Sep 2, 2007)

They don't address my questions. I want to know if a 60a controller would be OK for my setup or do I need to go with a 70a. And I would also like to know why 60a and 70a controllers all have such a high voltage input. I know I need an MPPT controller because the solar panels are 24v and the batteries are 12v, besides being more efficient.


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## wy_white_wolf (Oct 14, 2004)

MoonRiver said:


> They don't address my questions. I want to know if a 60a controller would be OK for my setup or do I need to go with a 70a. And I would also like to know why 60a and 70a controllers all have such a high voltage input. I know I need an MPPT controller because the solar panels are 24v and the batteries are 12v, besides being more efficient.


It also depends on the quality of controller you use. A cheap MPPT you need to calculate in the 25% as you did in your calculation because they have problems handling the high output production situations. Quality MPPTs like Outback can handle the extra power but may not send it to the batteries. To me that is not a problem as a properly designed system would be able to fully charge the batteries with out that power. High output condition generally happen less than 2% of the time. So it's more about getting the charge controller to survive than using the extra power.

So a quality MPPT charge controller - 50A minimum
Cheap MPPT charge controller - 62.5A minimum

Personally, I would go with a 60a quality controller as the cost difference between a 50a and 60a is not that much.

Higher voltage allows for the use of smaller wire and/or less voltage drop from the wire. With the higher voltage you can wire your panels in series producing a nominal 48v which would allow for wire 1/2 the size or less voltage drop from the wiring.

WWW


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## MoonRiver (Sep 2, 2007)

Thanks www. I understand plus it might save me $75.


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## Murby (May 24, 2016)

You need to look at the recommended charge rate of your lithium batteries. If you're buying consumer grade batteries, they probably have a charge controller or some kind of BMS system built right into them so that a consumer can treat them as a normal lead acid battery.

You need to figure out what the manufacturer recommends for a charge rate and then size your system accordingly.

I would stick to a quality brand and avoid anything made in China..


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## MoonRiver (Sep 2, 2007)

Murby said:


> You need to look at the recommended charge rate of your lithium batteries. If you're buying consumer grade batteries, they probably have a charge controller or some kind of BMS system built right into them so that a consumer can treat them as a normal lead acid battery.
> 
> You need to figure out what the manufacturer recommends for a charge rate and then size your system accordingly.
> 
> I would stick to a quality brand and avoid anything made in China..


They are BattleBorn 100Ah 12V LiFePO4 Deep Cycle Batteries. They also sell inverters and Victron MPPT charge controllers and will configure to work optimally with their batteries, plus they are on sale. The problem is they are so busy I can't get a call back. Thanks to www and a lot of research, I finally felt comfortable ordering the inverter and charge controller from their web site without talking to one of their techs 1st.


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## Gary in ohio (May 11, 2002)

Whatever you get make sure it will handle the lithium batteries.

Also not sure where you math is coming from,. It would better if you had the current but based on 300watts at 24v thats 12 amps or about 25 amps for both panels. SO max current is 25amps on the absolute best day.


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## MoonRiver (Sep 2, 2007)

Gary in ohio said:


> Whatever you get make sure it will handle the lithium batteries.
> 
> Also not sure where you math is coming from,. It would better if you had the current but based on 300watts at 24v thats 12 amps or about 25 amps for both panels. SO max current is 25amps on the absolute best day.


But the batteries are 12v, so that doubles amps going to battery. The amps for controller are the amps going to batteries. Input voltage is from the panels.

I'm buying the charge controller from the same company that makes the batteries. They only sell equipment that works with their batteries.


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## Murby (May 24, 2016)

I believe battle born batteries can be charged with pretty much any automotive charger since they have their own built in charge control and battery management systems.. This is why they call them "drop in replacements".. all the fancy stuff to protect the lithium is built right into the battery. 

As for charge controller, I would STRONGLY suggest you make sure any charge controller you get has the ability to set max volts AND max amps. Why? because when your battery dies or gets changed, upgraded, whatever, if you can't adjust the controller, then you'll have to replace that as well.

I don't have anything against Victron, seems like they have a decent reputation... that said, I think I would still go with a dedicated Midnight Solar or Outback charger.. or maybe even a TriStar as those are top of the line as well.

And don't even think of going with a PWM system, those are garbage and only meant for small hobby stuff like shed lighting or a chicken coop or something like a trickle charger.. Stick with the MPPT as they are FAR more efficient.. like comparing a 1970's Cadillac to a brand new Lexus.


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## Murby (May 24, 2016)

MoonRiver said:


> They don't address my questions. I want to know if a 60a controller would be OK for my setup or do I need to go with a 70a. And I would also like to know why 60a and 70a controllers all have such a high voltage input. I know I need an MPPT controller because the solar panels are 24v and the batteries are 12v, besides being more efficient.


If you plan on getting a second battery, you should really jump up to an 80 amp charge controller.. Pushing 55 or 60 amps through a 60 amp controller is going to make that thing work really hard.. leave a bit of breathing room.

Or, for redundancy, you could get two smaller controllers and have them work in parallel.


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## Itinerant1 (Jul 23, 2018)

Moonriver are you going wire the panels in parallel or series?
If series the voltage doubles (VOC?) the amp (IMP) stays the same.
If parallel the voltage remains the same, amps double.
The charge controller should be able to handle the voltage. If it has some programability its better for Lifepo4 batteries. Bulk minimal 14.2v (14.4-14.6 for cell balancing of those battery cells) little to no time in absorb, float 13.3-13.6v while you're actually using it. When storing for a period of time leave at a 50% soc. What will you use for battery monitor?


I personally don't have Battleborn batteries I have 500ah Elite GBS batteries (5 100ah batteries, 20 prismatic cells- Lifepo4) 1,280w (8 160w panels 2 in series, 4 set parallel), Magnum PT100 charge controller (100a), 3000w inverter. I fulltime in a 5th wheel traveling living off this system now 27 months.


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## MoonRiver (Sep 2, 2007)

Itinerant1 said:


> Moonriver are you going wire the panels in parallel or series?
> If series the voltage doubles (VOC?) the amp (IMP) stays the same.
> If parallel the voltage remains the same, amps double.
> The charge controller should be able to handle the voltage. If it has some programability its better for Lifepo4 batteries. Bulk minimal 14.2v (14.4-14.6 for cell balancing of those battery cells) little to no time in absorb, float 13.3-13.6v while you're actually using it. When storing for a period of time leave at a 50% soc. What will you use for battery monitor?
> ...


Since the panels are already 24v, I'm going to wire them in parallel.


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