Connecting Solar panels directly to battery bank

[kebang]

I’ve been told that you can connect solar panels directly to the battery if they do not exceed 2% of the batteries capacity. That is to say an 120watt array to a battery bank of 400amp/hr. Does anyone know if this is correct?

If it is correct in addition to the 120watt array can you also connect further array(s), via an MPPT controller, to the same battery bank?

kebang

[daz]

I’ve been told that you can connect solar panels directly to the battery if they do not exceed 2% of the batteries capacity. That is to say an 120watt array to a battery bank of 400amp/hr...kebang

Even if that is correct, that solar system would cost a fortune!!! Batteries are one of the most expensive parts of a solar system. Using a 2% solar array to battery ratio would mean you need a ginormous battery bank.....costing a lot of money!

The charge controller stops the batteries from receiving excessive voltage, as battery/charge voltage rises as the battery is charged. LA batteries have a maximum charge voltage, so you do not want to exceed it.

Is there a specific reason why you would not want to use a charge controller?

[kebang]

Thanks for the reply. I was trying to keep my post as short as possible.

I have a 400amp/hr 12v battery bank
I have 240 watt solar array with a 10amp MPPT controller.
I have a 2kw pure sine wave inverter
I have a mains powered battery charger

This system is not meant to be a 24/7 solar system! Its just a back up for brown outs when it runs my fridge, freezer, TV, video player & lights.

When there is no brown out on sunny days I run the house from the solar for 2hrs a day between 10a.m - midday.
After a 12 hour brown out I recharge using the mains charger.

Just wondering if it was okay to add another 120 watts of panels wired direct to the battery. I'm thinking if the battery is full the MPPT controller will disconnect the 240watt array and the battery will continue to be trickle charged directly by the new 120watt array. I'm thinking a max of 8amps (more likely less considering time of day, cloud etc} will not cook the batteries.

I could of course add further arrays using further controllers, but as this system is primarily for brown outs I'm trying to keep the cost down as much as possible.

kebang

[Mike90250]

for brown/blackouts when you have grid power, backup batteries and a mains charger for them, is the system to use, solar is way too expensive to use for such short-term charging. And if you wire a panel directly to batteries, you need a BLOCKING diode to prevent the battery from discharging into the PV array at night. A charge controller usually provides this function.

[kebang]

Thanks Mike, but as you can see from my second post, that is what I do.

I have the solar panels & solar charger listed above, just trying to get some use out of them! ((The 2 hours a day I mention above).

Unfortunately non of the responses I have received so far answer the question "Can I use an 120watt array without a solar charger in conjuction with a 240watt array with an MPPT controller'

kebang

[Sunking]

Thanks Mike, but as you can see from my second post, that is what I do.

I have the solar panels & solar charger listed above, just trying to get some use out of them! ((The 2 hours a day I mention above).

Unfortunately non of the responses I have received so far answer the question "Can I use an 120watt array without a solar charger in conjuction with a 240watt array with an MPPT controller'

kebang

No not without destroying the battery.

[SunEagle]

There are always those small wattage solar "trickle chargers" that you can use although they were designed for a 12volt car battery so will not really do much for your larger battery bank.

By the way. How did you build your 12volt 400Ah battery system? Hopefully it is 2 x 6volt 200Ah batteries wired in series.

[Mike90250]

Unfortunately non of the responses I have received so far answer the question "Can I use an 120watt array without a solar charger in conjuction with a 240watt array with an MPPT controller'

Yes, you can use it. (but you may not be happy in a month)

If you don't use a diode, then the battery will discharge into the array at night

I hope you have flooded batteries, and can add water to them, since 120W is more than a trickle charge. If you observe that the other charger has shut off when the battery is full, disconnect the non-controller panel to prevent ovdercharging, or turn on some loads to reduce the overcharge.

[billvon]

I’ve been told that you can connect solar panels directly to the battery if they do not exceed 2% of the batteries capacity. That is to say an 120watt array to a battery bank of 400amp/hr. Does anyone know if this is correct?

Sure you can do it; you will probably shorten the life of your batteries and/or require more watering but it will work. Lead-acid batteries can be overcharged as long as you add water as needed.

But why? You can get a cheapo charge controller for $30, and a decent one for $80. Probably save you far more than that in battery costs.

[PNjunction]

... Unfortunately non of the responses I have received so far answer the question "Can I use an 120watt array without a solar charger in conjuction with a 240watt array with an MPPT controller'

Technically yes, but operationally you'd be foolish to do so.

Consider that during the bulk charge stage, most controllers are essentially doing just that - attaching the panel directly to the battery. Once the battery terminal voltage rises to a preset value (or somewhat near it), pwm voltage control kicks in (be it inexpensive pwm or mppt controllers) during the absorb stage.

So in an emergency if your controller was fried, you *could* directly attach the panel across the battery, BUT now YOU are the charge controller, and you'd have to babysit the battery once it reached the absorb voltage. The battery would not be fully charged, but if you are on top of it, you could get up to about 80% SOC. Would you do this with a 120 watt panel to a 5ah agm? No, unless you had lightning fast reflexes to stop the charge in time. If you don't, you are stuck on Gilligan's Island with a smoked little battery.

[kebang]

Thanks to all who replied, much appreciated. I will not now be connecting any panels to the battery without going through a controller of some description.

kebang