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Old 28-02-2018, 01:48   #1
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Will a small solar regulator actually charge a house battery

From my reading (including Rod/Main Sail et al) I believe even 1amp at 14.4 volts will eventually top up a 120ah battery from 80 or 90% SOC. I have a 130ah US Battery house battery, a 40W solar panel (old so I assume now 20W) and a Genasun mppt solar regulator (4A).
I’m sure it will maintain the battery, but will it actually charge it?

Second related question, what can I do to force it into Absorbtion mode (i.e. above float voltage) as when I check it is always at 13.5 volts(float). Disconnecting panel or battery for a while does not seem to reset the ‘2 hr absorption mode’ timer.

We mainly do 3 to 5 days trips and occasionally 10, and remainder on a marina, but no shore power. I have installed a Victron BV700 which lets me see usage, but I don’t trust the SOC as we seem to draw down the battery quickly with only a 4A fridge load. I’m thinking the battery hasn’t actually been kept fully charged, even though the BM says 100%. And yes I check all the BM settings, am aware of the pitfalls.
Hope that makes sense?? Any help would be appreciated.
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Old 28-02-2018, 05:26   #2
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Re: Will a small solar regulator actually charge a house battery

You need a controller that will allow you to customize the charge profile, particularly the Absorb Hold Time, until it is refraining from dropping to Float until trailing current drops to your desired endAmps.

Setting Float at the same voltage as Absorb is another workaround if the solar day often ends before you get there.
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Old 28-02-2018, 09:08   #3
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Re: Will a small solar regulator actually charge a house battery

Let's say your fridge is insulated well enough that it runs 50% of the time. So your fridge can be calculated as averaging 2 amps. So in one day 24x2 you use 48 amp-hrs out of your battery.

An excellent day for your solar panel, would be full output for 5 hours. 5x1 is 5 amp-hrs. No shading, bright sunny day, etc.

48-5= 43 amp-hrs down. You don't want to draw down the battery more than 50%, so you only want to use 60 amp-hrs our of your battery.

So about in 1 1/3 days you've discharged your battery as far as you want to.
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Old 28-02-2018, 09:14   #4
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Re: Will a small solar regulator actually charge a house battery

Yes in less than perfect solar conditions even a very efficient fridge needs ~300+W solar and around 200+AH, pair of GCs for maybe $200 would do it.

Lower is OK if regular alt or genny charging.
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Old 28-02-2018, 10:29   #5
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Will a small solar regulator actually charge a house battery

Thanks for the input and I understand I should have a bigger house bank.
To be clear, I am not trying to run the fridge full time. It is just that the fridge draws the battery voltage down faster than I expect, given 50% cycle time, using 4A, so say over 12 hours ; 4 x 6 = 24 ah. The BM might show I have used 25% but the voltage is below 12.2V. I know it needs to rest etc. so...I just think the battery is MUCH less than the 130ah it started at. Hence trying to understand if the solar regulator is actually charging battery to full. (Even if that takes weeks).

So, with nothing running, back at marina, I want the small solar to charge the battery. Will it do that at float voltage, ever? Or can I force a GV4 to absorption? It has no such settings.

Incidentally I have added a second fan and more insulation to the fridge. Testing so far, it has reduced cycle time a lot!
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Old 28-02-2018, 12:24   #6
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Re: Will a small solar regulator actually charge a house battery

> It is just that the fridge draws the battery voltage down faster than I expect, given 50% cycle time, using 4A, so say over 12 hours ; 4 x 6 = 24 ah. The BM might show I have used 25% but the voltage is below 12.2V.

Every method for estimating SoC is a guesstimate, voltage most of all even with 48+ hours isolation.


> Hence trying to understand if the solar regulator is actually charging battery to full. (Even if that takes weeks).

You just need an ammeter showing the charge cycle.

1. CC/Bulk stage, V is low A are high

2a. CV/Absorb V limited to your selected setpoint, eventually A start to fall

2b. Amps fall to .005, 1/2A per 100AH

That is the best definition of Full, and only after that point is reached, should

3. V reduced to Float.

Float voltage usually means no more charging.

If the controller can't be configured to do that it should be replaced.

If the panels are sized so a full charge doesn't happen in a day or at most two, you need more panels, or don't discharge the bank more than the panels can produce.

When it's sunny and no loads, really should be one day.


> I am not trying to run the fridge full time

That is what they're designed for, takes at least 10-18 hours before most units start working efficiently, even if you pre-chill their contents.

Your electric system should be sized so it is comfortable running it 24*7 in suboptimal solar conditions


> I just think the battery is MUCH less than the 130ah it started at.

Quite likely, even just a few months of PSOC abuse, or pulling SoC too low can significantly reduce capacity, completely kill, even a top quality deep cycle bank costing thousands, within a year.


I'm not au fait with that Victron unit, the 702-BMV is the one I'd choose, but for SoC measurement nothing beats a SmartGauge.

Spend your money on more solar first 8-)
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Old 01-03-2018, 00:03   #7
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Re: Will a small solar regulator actually charge a house battery

your small panel possibly can't hit absorption voltage... it will only hit that when the charger is no longer needed to be maxed out and then starts to reduce current. at full current you are in bulk, and the voltage will be lower as it struggles. the MPPT controller, victron 700, and probably other things are taking a bit of the power too. so a bit less to charge with.

a bigger battery will not help you if you can't charge it.

either your battery is not full, or the battery is no longer any good. only way to find out is plug it into a charger for 24 hours. and try it again. if it drops to 12.2v at 25% down again then the battery is now weaker then new. if the battery stays higher after shore charging. then it was charged this time. and you need to find a better way to charge it each time.

there is no way you are going to do a 5 day trip on a 130ah battery with a fridge. unless you are running the engine a lot
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Old 01-03-2018, 00:39   #8
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Will a small solar regulator actually charge a house battery

Thanks John,
Understand all of that. The 700 is same as 702 without second batt voltage and 1 other thing. So I can follow A in and out.
And thanks Smac, I think you might have the answer, a small panel has not enough ‘power’ for the regulator to output an absorption level voltage. Hence if charging always at float level, I’m not charging the battery to full charge. I have taken the battery home, charged it repeatedly with a CTEK charger, ensuring it goes to absorption level (repeatedly) and also done equalization level charges. And yes it still drops too fast in use, so I think it has been undercharged for 2 years. To be clear I am not running the fridge for days on solar, we motor 2 hours per day so we are alternator charging up to 14.4 for some time, but to more like 2% rather than .5% of capacity rating.

We almost never go below 60% SOC and the history shows only one discharge below 11.5v

I had hoped that a small panel would top up the battery when back at the marina. I think the answer I was looking for is that the panel is too small for the regulator to output an absorption level voltage, hence the battery never gets charged, it only gets maintained at whatever level we leave it. (Even though the BM says 100%!)

Many thanks.
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Old 01-03-2018, 01:01   #9
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Re: Will a small solar regulator actually charge a house battery

"Getting to" Absorb is only 75-85% Full, even with a lot of current on mains, at least 4 more hours *holding* that voltage is needed to get to full.

Same with alt charging for just a few hours, even with a good VR setup just not enough time.

And yes two years of that abuse the batt is likely ready to be scrapped.

Use the ammeter monitoring above to try to care for the replacement bank better, you should be able to make it last 5+ years at least.

And yes if that's a 700-BMV you should be able to calibrate it so with frequent resets to Full it is within 10% accurate most of the time.
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Old 01-03-2018, 08:19   #10
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Re: Will a small solar regulator actually charge a house battery

Quote:
Originally Posted by daveNZ View Post
Thanks John,
Understand all of that. The 700 is same as 702 without second batt voltage and 1 other thing. So I can follow A in and out.
And thanks Smac, I think you might have the answer, a small panel has not enough ‘power’ for the regulator to output an absorption level voltage. Hence if charging always at float level, I’m not charging the battery to full charge. I have taken the battery home, charged it repeatedly with a CTEK charger, ensuring it goes to absorption level (repeatedly) and also done equalization level charges. And yes it still drops too fast in use, so I think it has been undercharged for 2 years. To be clear I am not running the fridge for days on solar, we motor 2 hours per day so we are alternator charging up to 14.4 for some time, but to more like 2% rather than .5% of capacity rating.

We almost never go below 60% SOC and the history shows only one discharge below 11.5v

I had hoped that a small panel would top up the battery when back at the marina. I think the answer I was looking for is that the panel is too small for the regulator to output an absorption level voltage, hence the battery never gets charged, it only gets maintained at whatever level we leave it. (Even though the BM says 100%!)

Many thanks.
On the original question assuming no draw on the battery and a battery in good condition yes the solar should at some point reach 100% charge to the batt. Even with the tiny amount of solar and loss from the charger you should hit float (if you leave it long enough).

My really awful back of hand math. (I used 12V for simplicity) and ignored efficiency issues)
1440 WH batt pack
20 Watt solar 4 hours a day full output
80 WH input per day
Self discharge of pack (based on 5% per month and 30 days)
2.5 WH per day
Consumption of solar charger (from morning star website)
10.8 WH per day.
So in theory you should be adding 66.7 WH a day. Of course days with no sun and clouds well mess this up. And it assumes no draws like a stereo memory exist. Also if the battery is poor quality or has other issues the self discharge goes up.
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Old 01-03-2018, 09:05   #11
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Re: Will a small solar regulator actually charge a house battery

Yes and no... use enough panels and 2 PWM chargers for starboard + port and save energy wherever you can. i.E. i have 2,8 m² panels and th eenry is good for 2 weeks with refrigerator, navigation, computer and autopilot (40% sailing, 60% on engine)
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Old 01-03-2018, 10:39   #12
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Re: Will a small solar regulator actually charge a house battery

We use a small solar panel to keep your 225ah house bank charged while on the mooring. it is a Solbian 23W All-in-One Flexible Solar Panel and does a great job.
Yes it will bring the house bank from 90% up to 100% in a day or two.
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Old 01-03-2018, 11:43   #13
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Will a small solar regulator actually charge a house battery

Guy(?),
I had hoped my (ex 40Watt) panel would do the same. My question is ‘why does the solar regulator not go to absorption mode’. One opinion is that there is not enough power from the panel. So Assuming my battery is not fully charged, why does it charge at 13.5 (can’t remember exactly) float voltage, and 1 to 2 amps? Should it not be trying to charge at 14.x at least, even if that means .5 A. (VOC - Open circuit V for my panel is over 20V)
Does your Solbian panel change at absorption level?
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Old 01-03-2018, 11:43   #14
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Re: Will a small solar regulator actually charge a house battery

But only if the Absorb voltage is maintained.

As soon as voltage drops to Float the current will stop flowing long before the batt is full.

This is easily verified using an ammeter as outlined above.
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Old 01-03-2018, 11:45   #15
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Re: Will a small solar regulator actually charge a house battery

We overlapped comments there. Even at float V my regulator still shows over 1A if sunny. Is that not really charging the battery?
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