What does "Maximum Discharge Current" of a Charge Controller mean?

[sheep]

Hi all.
I came across this Forum while Googling for (and failing to find!) an answer to this question.

I'm looking at the specifications for various Charge Controllers to use with an off-grid solar system, but a lot of them seem to quote a value for the "Maximum Discharge Current" and it's often the same number as the maximum charge current (i.e. Max Charge Current = 20A, Max Discharge Current = 20A).

What does this number mean? Is it the maximum current that can be drawn from the Load output of the charge controller? If so, 20A at 12V isn't much use! But I can't think what else it could mean!

Can anyone help?

Thanks!

[Naptown]

Post a link to one you are looking at.

[Sunking]

Sounds like you are looking at Chi-Com equipment.

Charge controllers are rated for their maximum safe current that they can deliver or pass through to a load like a battery. For example a 20 amp controller means just that, it is rated up to 20 amps. However the controller does not make the current, the current is determined by the solar panels, not the controller per sei.

[sheep]

I'm mostly looking on eBay, and nearly all of them seem to quote a "maximum discharge current".

Here are a couple of examples from the Web:

http://linksolar.en.alibaba.com/product/725188279-214689389/low_price_5A_12v_font_b_solar_b_font_charge_controller.html

Maximum charge current: 1A
Maximum discharge current: 1A

http://1013668.en.makepolo.com/products/low-price-led-solar-charge-controller-p32819610.html

Maximum charge current: 3A
Maximum discharge current: 3A

Just google for solar charge controller "maximum discharge current" and look at pretty much any result.

[Sunking]

Just what I thought, you are looking at Chi-Com importers. Charge controllers are rated for maximum output current. What else is there to say?

[sheep]

Sounds like you are looking at Chi-Com equipment.

Is there anything else?

Charge controllers are rated for their maximum safe current that they can deliver or pass through to a load like a battery. For example a 20 amp controller means just that, it is rated up to 20 amps. However the controller does not make the current, the current is determined by the solar panels, not the controller per sei.

Hmmm. But surely the idea of the Charge Controller is to regulate or "control" the battery charging, so limiting it to 20A (or 5A or whatever) is a function to protect the battery, not a "rating" for the equipment? I would expect the maximum load you could draw from the battery to be substantially higher than the charge rate.

[sheep]

Just what I thought, you are looking at Chi-Com importers. Charge controllers are rated for maximum output current. What else is there to say?

I thought the main function of a charge controller was to restrict the charging current passing through the battery to avoid damaging the battery. So it's not a "rating", it's actually the main function of the equipment to limit the current passing through the battery.

Can you point me to a Charge Controller that isn't chicom?

[Sunking]

Is there anything else?


Hmmm. But surely the idea of the Charge Controller is to regulate or "control" the battery charging, so limiting it to 20A (or 5A or whatever) is a function to protect the battery, not a "rating" for the equipment?

The controller only regulates the voltage by sensing it and turning the controller off when the battery is charged up. How much current that is supplied is strictly the function of what size panel the panel is. Let's say you have a panel capable of 100 amps and attach it to a 20 amp controller, the controller will burn up when it turns on to charge a battery. Panels, controllers, and batteries all have to be matched up to work with each other.

[sheep]

The controller only regulates the voltage by sensing it and turning the controller off when the battery is charged up. How much current that is supplied is strictly the function of what size panel the panel is. Let's say you have a panel capable of 100 amps and attach it to a 20 amp controller, the controller will burn up when it turns on to charge a battery. Panels, controllers, and batteries all have to be matched up to work with each other.

Ah, right! I think there are two types of charge controllers, and we're talking about different things here

The units I'm thinking of are more intelligent than the type you describe, and do proper voltage regulation (and current limiting) to charge the battery. They take a high-voltage input from the PV panel, and drop it down to ~14V to charge the battery. I hadn't realised there was a simpler type with no voltage regulation. Now I know that, things are beginning to make a bit more sense!

Still confused as to exactly what "max discharge current" means - I suspect something may have got lost in translation from Chinese there.

[Sunking]

Ah, right! I think there are two types of charge controllers, and we're talking about different things here .

No we are on the same page. I am talking both low end PWM chargers you are looking at from China, and High End MPPT charge controllers made in the USA and other countries. Current is controled by selecting the right amount of panel power. That is part of the design process.

[russ]

Can you point me to a Charge Controller that isn't chicom?

You lack an understanding of international business and manufacturing. Good equipment is made in China as well as cheap garbage. Companies with a name work to protect that name.

No name companies have nothing to protect and care little or not at all. You are talking about no name brands as if they were useful.

[sheep]

No we are on the same page. I am talking both low end PWM chargers you are looking at from China, and High End MPPT charge controllers made in the USA and other countries. Current is controled by selecting the right amount of panel power. That is part of the design process.

There is another type of controller I've come across which apparently does nothing more than interrupt the connection between the battery and the PV panel when it decides that the battery is fully charged. It does no voltage regulation or current limiting. Example: http://www.maplin.co.uk/solar-charging-regulator-37774

I think both of the types you describe (PWM and MPPT) do voltage regulation - the main difference is how they interact with the PV panel.

Incidentally, I think I have pretty-much worked out the answer to my original question - they seem to use the term "Discharge" to mean "Load". So when they say "20A max discharge" it really means "20A max LOAD".

Thanks for your help!

[inetdog]

Incidentally, I think I have pretty-much worked out the answer to my original question - they seem to use the term "Discharge" to mean "Load". So when they say "20A max discharge" it really means "20A max LOAD".

Your understanding looks good so far. Some CCs also have LOAD terminals which may have an output limit lower than the maximum charging current output. They are used mainly for time or sunlight controlled lighting or for small loads that should only be powered when the batteries are above a certain voltage level. But do not try to generalize to any other manufacturer or situation. With a battery, for example, charge and discharge are just what you would expect and the current limits for charging will vary with state of charge and temperature, while the discharge current limits will depend on the internal resistance of the batteries and just what your power needs are. Discharging too quickly will give you less stored energy than you would get at a slower rate, but if it gives you what you need and your batteries will still last a reasonable time, that is your option.

I think that you are correct in your understanding of what "Discharge" means for the CC you are looking at. As you noted, it would make little sense to connect and inverter to the LOAD terminals of the CC rather than directly to the battery. The inverter will have its own low voltage cut off (LVCO) to protect the batteries although the default value is often way too low.
Any time control of specific loads will be done at the device, on the output side of the inverter.