Battery Charge State

Stu Jackson · Jun 11, 2011

...it cuts in, assuming the panel is outputting, whenever the batteries are below 13v, and cuts out when they reach 14.2v. I sat there with a multimeter on the main bus and saw it cut out and went from charging to charged at 14.17v...

Brian,

It appears that this charging regimen is not complete, since the bank is not seeing a continuous charge of any kind for any reasonable time period at a 14.2V level, which is required for bulk charging to get to the absorption phase (or stage). I'm sure, from your contributions here on this board, that you're familiar with the details of those three stages or proper charging, so I won't repeat them here.

Essentially, you're almost "getting there" but when the controller drops back to the 13+V level, you're not getting a either of the full bulk or absorption phases, just going back to float, then starting all over again. It seems to me that that is why you're not ever seeing a full battery charge.

Am I risking damage / sulfation to my batteries because the charge controller isn't actually charging the batteries to a high enough voltage (I've read anywhere from 14.5-14.8 is often considered fully charged), or is the built in hydrometer not reliable?

Yes, 'cuz your undercharging your banks by not ever fully charging them. It's not so much charging them "to a high enough voltage," but rather charging then AT a high enough voltage to push the amps back in. Those voltages you mentioned are NOT what is considered "fully charged" since those voltages should eventually appear when you first start charging, and stay there until the amperage going into the bank starts dropping. Here's West Marine's explanation of the three stage charging regimen: http://www.westmarine.com/webapp/wc...d=11151&catalogId=10001&page=Battery-Charging

You can also read more in the Ample Power Primer, www.amplepower.com.

I'm sure you'll find a good recommendation for an MPPT controller if you do a search on that using Maine Sail as an author.

Good on you for noticing that. I guess you get what you pay for...

Maine Sail · Jun 11, 2011

What you have is a simple shunting or switching regulator that basically does what you've seen, turns on and off when it hits its target voltage.. These will charge the bank to full but it takes a very LONG time even with a large array. With a small array it can significantly add to the time it takes to get to a full charge.

The combination of a panel that puts out a max current of just .78A on-top of a simple on/off regulator means your batts are going to take a very, very long time to charge.

If you figure max current of .78A for about 5 hours per day, and that is the general solar consensus, you're only putting back 3.9 Ah's per day in a best case scenario.. Another well accepted output measurement is 42% of max rated output for 12 hours of sun per day. So 12 X 0.78A = 9.36A/Day at 42% =3.9A/Day and you basically come out the same as the 5 full at max number.

If you have say a 300Ah bank at 80% SOC, an easy point for an alternator to re-charge to, that means you'd need to put back 60 Ah's to be full. However, due to charging efficiency losses, you more likely need about 70 Ah's to really get closer to full.

Simple math shows that with a need for 70Ah's @ 3.9 per day means it would take you nearly 18 days to reach a full charge. This math of course does not include ANY overcast or rainy days so you could be well over a month to replace 70 Ah's..

Also, I did not account for the shunting regulators slowness or the batteries natural self discharge which may mean your barely keeping up with the self discharge and any phantom loads like a stereo memory let alone actually putting excess charge back into the battery...

You have a solar array that can perhaps maintain the battery but will not very effectively charge it, and certainly not in two days, unless you left it already sitting at 99% SOC.

A true 3 stage solar controller may help some but not a lot because the numbers are what they are. The Morningstar controllers are excellent true three stage devices and some of the best built. They are reliable and priced fairly for what you get. Like LED bulbs there is a lot of junk out there in terms of cheap solar controllers. Morningstar is not cheap junk. A good controller may get you to at least your 5 hours/day @3.9 Ah's averaged but won't and can't work magic.

An MPPT for a 12W panel is like putting a Ferrari engine in a Yugo and you really don't have enough excess voltage in that panel to even get an MPPT to work and do what it would need to do so that would be a waste of money.

A 40W monocrystalline or polycrystalline panel, often called multicrystalline, is about the minimum I'd recommend if you actually want to re-charge in any decent amount of time. To do it in a week you'd likely want to go bigger. The thin film panels, the flexible kind, are about 50% less efficient than a mono or polycrystalline panel.

Was working on a boat yesterday with a 40W monocrystalline panel and she was putting out about 2.2A at mid day sun and was not using an on/off charger so she had a steady absorption current and was not yet to float stage. A 40W poly or mono panel will cost about the same as the thin film panel but take up slightly more room and will not be flexible.

Sorry to not have a magic bullet. If you upgrade to a Morningstar 3 stage controller it can be used with a future panel..

Maine Sail · Jun 12, 2011

FourPoints said:
forgot to add, the panel outputs around 17.5v at midday, and around 15-16v at morning or late afternoon, not sure what is considered excess voltage for this scenario

I know the image on defender's site is misleading, and it looks like a flexable panel, but it's actually a polycrystaline panel, just a very thin and rugged one.

If your panel is the VPC-12 the Vpm on your panel is rated at 15.3 volts. The OCV or Voc reading is higher at 19.3V but that is a voltage reading with no loads and not one you'll see in real use with any load connected to the panel. Your max amp output is .78A.

For an MPPT to work well you really need a lot more watts than 12 and some decent excess voltage that can be sustained by the panel when connected to the load.

The only MPPT that might aid in your charging are the Genasun controllers as they are specifically built MPPT's for small panels and they are reasonably priced. The others by Blue Sky, Morningstar, Outback and others are built for larger arrays.

I still don't know that roughly 2.7 volts, if you can sustain it with a 130Ah bank, is going to convert to a meaningful amp output increase with an MPPT. Even if you could get 15% more amperage from an MPPT your still at only .1A more output.

Do you have a battery monitor?? That would be a good expenditure before buying another charge controller...

SailingHarry · Jun 13, 2011

I have heard at various times, but can't quote anything to back me up, that at some "very low" solar output level, a complete lack of controller is not inappropriate. I wonder if this is such a case? The panel is good for 3.9Ah max, and the battery is 130Ah, making the daily input only 3% of capacity before shade, charging losses, self discharge, etc.

Realistically, how much damage would be done if it were direct wired? And would it be, on balance, better than the existing controller (or any other controller that costs less than a new set of batteries)?

Harry

Ron20324 · Jun 21, 2011

SailingHarry said:
I have heard at various times, but can't quote anything to back me up, that at some "very low" solar output level, a complete lack of controller is not inappropriate. I wonder if this is such a case? The panel is good for 3.9Ah max, and the battery is 130Ah, making the daily input only 3% of capacity before shade, charging losses, self discharge, etc.? Harry

Page 368 of the West catalog/bible offers that if a charge is more than 1.5 % in Ah of the battery capacity, you need a controller.