Parallel charging

mmeager

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Sep 12, 2021
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Hi, wondering if you can help, as I've had conflicting information given. I have two small AGM 12ah batteries connected in parallel to make a 24ah battery box primarily used to run camping LED lights. Both batteries are Full River DC 12-12. The specs for each battery states recommended charging at 3amps, max charging at 4.1amps. But if batteries are in parallel, does this mean that I can safely charge at 6amps? Or should I still limit charging at the recommended 3amps as if it were a single battery? Reason being I would be charging via solar when off grid, obviously a 6amp charge rate would be better, however I don't want to cause any damage or reduction in battery life. Another reason is I'm finding it difficult to find a PWM controller limited to 3amps that would be suitable for a solar panel above 40w, mine will be 200w. The Victron PWM 6amp controller however would be suitable.
 

MurphyGuy

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A 3 amp charge at 24 volts = 72 watts going into the battery.
A 6 amp charge at 12 volts = 72 watts going into the battery.

It doesn't make any difference if you charge them at 3 amps in series (24 volts) or at 6 amps in parallel (12 volts)
 

mikefitz

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Another reason is I'm finding it difficult to find a PWM controller limited to 3amps that would be suitable for a solar panel above 40w, mine will be 200w. The Victron PWM 6amp controller however would be suitable.
With most PWM controllers the current limit is controlled by the solar panel, a 200 watt '12v'solar panel with a max power voltage of 18 volts will supply around 10 amps with good solar conditions. Any solar regulator must be capable of carrying this current.
Victron PWM controllers are either 5A (or up to 60 watt solar panel) or 10A (up to 120 watts with a 12v system)
Your two 12Ah batteries in parallel make a 24Ah battery. The recommended 3A per 12 Ah is about right for an AGM battery, so a charge current of 6 amps is OK for the two batteries in parallel.
Its interesting that Victron recommend only a 36 cell 120 watt panel for a 10A PWM controller.

Assuming you are using the two batteries in parallel, then you are limited to using a 36 cell 120 watt panel and the Victron 10A PWM controller, this will produce in the region of 6 amps into the battery.

Mike
 

mmeager

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Sep 12, 2021
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Thanks MurphyGuy and Mike for your answers, that's exactly what I thought, I just had some previous conflicting advice, and wanted to clear that up.
 

mmeager

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With most PWM controllers the current limit is controlled by the solar panel, a 200 watt '12v'solar panel with a max power voltage of 18 volts will supply around 10 amps with good solar conditions. Any solar regulator must be capable of carrying this current.
Victron PWM controllers are either 5A (or up to 60 watt solar panel) or 10A (up to 120 watts with a 12v system)
Your two 12Ah batteries in parallel make a 24Ah battery. The recommended 3A per 12 Ah is about right for an AGM battery, so a charge current of 6 amps is OK for the two batteries in parallel.
Its interesting that Victron recommend only a 36 cell 120 watt panel for a 10A PWM controller.

Assuming you are using the two batteries in parallel, then you are limited to using a 36 cell 120 watt panel and the Victron 10A PWM controller, this will produce in the region of 6 amps into the battery.

Mike
Mike does this mean I can't run the 10a pwm from a 200 or 300w panels?

I'm just getting everything together atm, so haven't actually purchased anything in regards to solar. I've sorted out my power consumption needs, and our main power source is a 120ah agm. The little 24ah battery box I made up myself originally as a learning curve, but it turned out so well I made a second one! Just a really handy little power source easily carried around to run various things such as lights, ph charging, small pumps, backup for fridge etc. Now I just have to source suitable panels and controllers to cover the charging needs of both the 120ah battery and the the two little 24ah battery boxes.

At present we will only be doing short week maybe 2 week off grid camping trips. My idea is to get practical real time experience and learning from this set up, because in 5yrs the wife and I are planning a 1yr camping trip around Australia, in which case everything will be modified according to what I have learned from my current planned set up.

I have spent the last 6 months researching, have learned a lot, but there's still so much to learn!!! And I'm enjoying every bloody minute of it!!!!
 

mmeager

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I should add in here, this is probably a little overkill! And not essential, but I also thought of running a stand alone 12ah agm of the same type (Full River DC 12-12) purely to run 1 LED bar light in a toilet/shower ensuite tent. So it would really only be pulling 0.6a at nights when toilet or shower is used. But this would also need to be added to my charging needs.
I did source a small 3a pwm, which I thought of using a small 20-40w solar panel as a permanent fixture for this battery alone, I figured that would be enough to cover the very small power consumption used.
Your thoughts?
 

chrisski

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Mike does this mean I can't run the 10a pwm from a 200 or 300w panels?
If you look in the PWM owners manual it usally has A voltage input range from about 18 volts to 22 volts for a 12 volt chargino system to 33 to 40 volts for a 24 volt system.

A single 100 watt panel hits the 12 volt PWM fine, and to get more watts, need parallel. Two 100 watt panels in series will charge a 24 volt PWM, and from there parallel them.

Some 200 watt panels and more 300 watt panelbs usually do not fit the voltages for a PWM panel either for 18 or 24 volts.

If you decide on an MPPT SCC< you have a lot more flexibility.

I did source a small 3a pwm, which I thought of using a small 20-40w solar panel as a permanent fixture for this battery alone,

That will work to charge a 12 ah battery provided the input voltages are met. The one caveat is this has to be out in the sun during the day to work. For my remote camping I’ve found shading can kill production. Specifically when I set stuff up in the shade to stay cool. Bad Idea for solar. Also, if this light were not turned off, you may start to cut into storage capacity after a time or two.
 

mmeager

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If you look in the PWM owners manual it usally has A voltage input range from about 18 volts to 22 volts for a 12 volt chargino system to 33 to 40 volts for a 24 volt system.

A single 100 watt panel hits the 12 volt PWM fine, and to get more watts, need parallel. Two 100 watt panels in series will charge a 24 volt PWM, and from there parallel them.

Some 200 watt panels and more 300 watt panelbs usually do not fit the voltages for a PWM panel either for 18 or 24 volts.

If you decide on an MPPT SCC< you have a lot more flexibility.



That will work to charge a 12 ah battery provided the input voltages are met. The one caveat is this has to be out in the sun during the day to work. For my remote camping I’ve found shading can kill production. Specifically when I set stuff up in the shade to stay cool. Bad Idea for solar. Also, if this light were not turned off, you may start to cut into storage capacity after a time or two.
Thanks for taking the time to reply chrisski, appreciate your input.

My understanding was that victron's pwm range from 5 to 30a is capable of 28v from solar input.

Originally mppt was my choice, and I will definitely be using a mppt controller for my 120ah agm. However the smallest mppt I could find was 10a output, I understand that real life applications may never achieve that 10a output, not constantly anyway, but if it does, and depending on my choice of solar panel, I was afraid this would be too much for the little 24ah and may damage it. That's why my thoughts went down the pwm controller.
So my question would be will a mppt provide too much current for the 24ah agm?

The little 12ah battery box for toilet/shower will have a main switch isolating the battery from any load, as do my other battery boxes, the charge input bypasses that switch and runs directly to the battery, which I would fuse at 5a for the 12ah. So essentially the battery would be switched off throughout the day. At night the LED bar light has it's own on/off switch, so the light would not be permanently on. So let's assume that between the two of us the light would run for around an hour intermittently per night. I figured due to dod the 12ah becomes 6ah. So 6/0.6=10 so either 10hrs of constant run time or 10 days of 1hr per night.
Have I got that right!!!??
We would mostly shower during the day, but that would give us ample room if we needed to use the light a bit more during the night. I was thinking a little 40w solar panel, that we could move around would be enough to keep it charged.
 

mikefitz

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So my question would be will a mppt provide too much current for the 24ah agm?
Any solar controller PWM or MPPT will take power from the panel and send to the battery. The MPPT takes all the power from the panel and converts into power to the battery. The PWM is less efficient, because all it does, in simple terms is connect, the panel to the battery. Thus the battery gets panel current.

example a 100 watt 36 cell panel specification (typically) is maximum power 18 volts, 5.5 amps, 100 watts.

assume the battery charging voltage is 14 volts, ideal solar conditions, no losses in cables and controllers.
MPPT controller current into the battery, 100/14 = 7.1 amps
PWM
controller current into battery, 5.5 amps

Of course this performance is under ideal conditions and 100% efficient controllers in practice perhaps 80% of those values would be seen at local noon
Also you have to consider the solar yield varies through the day.

With your battery you only have to worry about the maximum current, as the battery is charged, the current will reduce as the battery internal resistance increases.

At a guess a 100 watt panel will put a minimum of 15 to 20 amp hours into the battery over a sunny day.

Based on your calculations a 100 watt panel would be more than enough, will not provide over current to the battery and still provide enough is poor solar conditions to meet demand. It perhaps depends of practically of storing and deploying the solar panels.

It may be there are alternative solutions to having separate controllers and panels for the various batteries and different charge sources. For example connecting the batteries in parallel and using a single charge source.

Mike
 
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