Is it possible to charge a small capacity battery with larger spec panel and charge c

[Zionosis]

So if I have a 200w panel and a 20amp mppt charge controller would that not put too much power into say a 20AH 12v battery.

I mean I read that you generally should charge a battery at a quarter it's capacity. So a 100AH battery would be charged with no more than 25amps.

I know that high voltage causes gassing but also too many amps cause overheating and ruin the life of the battery.

So if I connected my 20AH battery to the 20amp mppt controller which is connected to a 200w panel would it pretty much be a bad idea since the mppt drops the voltage to 13.6 or so which is fine but it also raises the amps to preserve the power of the panel and would feed around 14amps into the battery which would be far too much.

According to the batteries AH rating I should only charge it at 5amps actually. But you can't hook up a 5amp charge controller to a 11amp or so large solar panel right? I am pretty sure you have to match it or you ruin the charge controller.


So what is the best way of charging a smaller battery.

[Mike90250]

Some of the fancy controllers can be programed to deliver only a limited (max) amps. Or you can use a smaller wattage panel which will give less amps.

Or aim the panel a bit west of south (South west) it will slowly deliver power the first half day at less than it's nameplate, and by early afternoon, the battery *may* be full enough, that it won't take full power from the panel. (But this is just a guess)

[jony101]

I have a 240 watt panel connected to a 20 amp mppt charge controller, even in full sunlight i never see more than 12 amps. The mppt will drop the voltage to about 14.4 volts and your battery depending on charge will determine the amps. A fully discharge battery will demand more amps from the mppt controller than almost a fully charge battery. And you do need to see over 14 volts when the charge starts to get a full charge, thats whats recommended for all lead batteries, 13 volts is only for float/maintaining.

I have charged my jump starter pack from the mppt controller directly (about 17 ah agm battery) without any problems. Unless I'm using the pack as I charge it, i always disconnect it when fully charged. As the battery gets full, the mppt will switch to float mode and the voltage drops to 13.7 volts and the amps going to battery gets tapered off to less than an 1 amp.

Mod note - Really poor advice follows - Myself i would have no problem charging a 20 amp battery with my mppt controller, if anything the battery would get an extremely good charge from the mppt.

[russ]

if anything the battery would get an extremely good charge from the mppt.

Pray tell wtf an "extremely good charge" is.

[PNjunction]

According to the batteries AH rating I should only charge it at 5amps actually. But you can't hook up a 5amp charge controller to a 11amp or so large solar panel right? I am pretty sure you have to match it or you ruin the charge controller. So what is the best way of charging a smaller battery.

First, determine the chemistry. If it is flooded, then the usable charge range is from about C/12 to no more than C/8 - where "C" is the Ah rating of the battery (usually spec'ed at the 20-hour rate)

For AGM, they typically spec about 0.25 to 0.3C max charge current, but there are others that will handle more. Yours sounds like the more typical type, ie 20ah * .25 = 5A max. Yes, you can charge faster but at the cost of service life, and thermal runaway issues especially if the battery is not quality.

Also check the maximum absorb voltage rating for the battery, and make sure your controller is not set any higher than it needs to be. For agm, that is typically 14.4 to 14.7v depending on manufacturer.

[PNjunction]

Myself i would have no problem charging a 20 amp battery with my mppt controller, if anything the battery would get an extremely good charge from the mppt.

I would. The typical AGM battery inside jump starter packs are not the world's best quality, and I would limit them to no more than 0.25 to 0.3C charge rate - which if you take the pack apart is usually seen listed on the battery itself.

For emergency purposes sacrificing cycle life with these ups-style agm's, you can get away with more charge current, but I don't recommend it unless you are in a total jam for time.

[inetdog]

According to the batteries AH rating I should only charge it at 5amps actually. But you can't hook up a 5amp charge controller to a 11amp or so large solar panel right? I am pretty sure you have to match it or you ruin the charge controller.

If your CC is PWM, then no, you cannot over panel them by more than the manufacturer allows, probably 10% or so. The reason is that although the pulse width modulation can still regulate the average current to the battery, the peak current during the on pulse will be too high for the switching element(s) and they will fail.
If your CC is MPPT, most will just regulate to a lower power level than the MPPT point to keep the output amps down. You may be able to overpanel by a factor of two or more without problems.

[PNjunction]

Happy new year Inetdog!

Remember that a charge controller, be it pwm or mppt only controls the upper voltage absorb limit, and not the current. The battery itself is the one controlling the current - within reason by means of the battery terminal voltage rising to the absorb voltage limit of the controller or charger.

During bulk, be it pwm or mppt, it is like having a direct connection without the controller inline at all. Mppt does bulk more effectively than pwm by virtue of the up/down conversion of voltages during this stage. However, during absorb, both a simple pwm and mppt controller use pwm for controlling the upper voltage for absorb. At this absorb stage, the battery is doing self-limiting of current all by itself - limited by the voltage differential. Of course if one didn't have a controller at all, the battery would try to rise to 18v, (typical for a 12v nominal panel) having no voltage-differential as a limiting factor to start the absorb process.

So one wants to choose the panel current output carefully to make sure it doesn't exceed the battery charge current specs during bulk, as neither controller has anything to do with current - just voltage.

In my case, I use a simple Morningstar pwm charge controller with the jump starter pack and dinky 80w panel - about 4.5A during the best of times in bulk. This works ok for the typical 18-22Ah agm inside - but I wouldn't go much higher in panel output. I carefully lay out the jumper leads, enable the leads and make them active, and attach the Morningstar to those *first*, and then attach the panel to the controller last. Once it reaches absorb, the battery does the actual limiting.

Note that since we are already in absorb current-limiting, and the chopping pulses are so very fast (something like 30k to 300mhz - I forget), that no harm is done to the battery with pwm - be it the simple pwm only, or mppt that actually uses pwm techniques during absorb too.

[inetdog]

Happy new year Inetdog!

Remember that a charge controller, be it pwm or mppt only controls the upper voltage absorb limit, and not the current. The battery itself is the one controlling the current - within reason by means of the battery terminal voltage rising to the absorb voltage limit of the controller or charger.

Thanks for the clarification. I was under the impression that most controllers, and all MPPT CCs also limit current to their design value when in bulk.
I believe that some of the more sophisticated MPPT CCs even allow you to specify the maximum bulk current at any point below the design level.
Small (5A, 10A) CCs are not my strong point.
And with an MPPT CC it is probably easier and uses fewer components to offset the input operating point away from MPP to limit output current or voltage than to add PWM into the output circuit.

[Mike90250]

PWM has no way, except it's internal fuse, to limit amps output. That's why they cannot be used to charge a 12V battery from a 24V battery.

MPPT is a computer controlled DC-DC converter, and CAN limit output power, by software/firmware setting, and thermal rollback to keep the components safe. How much control the user has, depends on the brand/model.

[inetdog]

PWM has no way, except it's internal fuse, to limit amps output. That's why they cannot be used to charge a 12V battery from a 24V battery.

MPPT is a computer controlled DC-DC converter, and CAN limit output power, by software/firmware setting, and thermal rollback to keep the components safe. How much control the user has, depends on the brand/model.

PWM cannot limit peak current, but it could limit average current. But a simple PWM controller is not likely to have the additional logic to perform that average current limiting.

The specifications for PWM CCs generally do not allow for any oversizing of the panel beyond the rated current of the CC, while MPPT CCs often specify a 1.5 or 2X overpanel range. The instantaneous current into the input stage during the voltage conversion is probably the reason that there is a limit.
But that is relative to design input current rating, not relative to a user-set maximum output current. So you should have not problems with a panel array that can drive the CC to maximum output in bulk but limiting that current to a lower value in software.

[LETitROLL]

Thanks for the clarification. I was under the impression that most controllers, and all MPPT CCs also limit current to their design value when in bulk.

Similar to the OP's original question, the above statement is something I have been looking for an answer to. For example does anybody know if the average MPPT controller would pass more current to the outputs than its max rated amp spec. (10a, 20a etc.) To further clarify, a 235w panel 7.8a, 24v, to a 10a MPPT controller to a 12v battery, i assume 235w / 12v = 18+ amps to batt, but will a 10a MPPT controller ever allow 18 amps to flow out of its 10a max output?, the 10a controllers I have looked at mostly show input 10a, output 10a, load 10a for max values, with acceptable input range and load connections not to exceed 10a, will it manage the output (to 10a or less), or do I have to do that in my design?
thanks for any insight

[SunEagle]

Similar to the OP's original question, the above statement is something I have been looking for an answer to. For example does anybody know if the average MPPT controller would pass more current to the outputs than its max rated amp spec. (10a, 20a etc.) To further clarify, a 235w panel 7.8a, 24v, to a 10a MPPT controller to a 12v battery, i assume 235w / 12v = 18+ amps to batt, but will a 10a MPPT controller ever allow 18 amps to flow out of its 10a max output?, the 10a controllers I have looked at mostly show input 10a, output 10a, load 10a for max values, with acceptable input range and load connections not to exceed 10a, will it manage the output (to 10a or less), or do I have to do that in my design?
thanks for any insight

I see two issues with using a 10amp MPPT controller with a 235watt panel.

First is that the CC may malfunction (blow an internal fuse, or just smoke) because it is trying to put out more than it's maximum amp rating.

Second. If the CC has internal protection to limit the output to 10amps no matter what the input wattage is, you will be turning that 235watt panel into a 120watt panel. Seems like a waste of pv wattage to me.

Simple answer would be to get a 30ampr rated MPPT charger which would allow you to use that 235watt panel and not push a smaller CC to it's limits.