Poor Man's Solar

engnrsrule · 07-01-2021, 12:29 AM

I considered various levels of Solar as a way to extend boondocking. I had bought a small solar battery tender to help keep my Toad battery from going dead while towing. Since adding a battery tender connected to the motor home umbilical, I wondered if the little solar panel would be of any use on the Roadtrek.

On our last trip I boondocked for 6 days. I used coach batteries for fridge controls (cooling running on propane), lighting, ceiling fan and maybe watching a movie at night. My batteries would be between 85-90% by morning. With the panel on the dash and plugged in to an accessory jack the coach batteries were 100% by evening.

If you are not a heavy power consumer this might be a good option.

https://www.amazon.com/dp/B07DFCDKLJ

peteco · 07-01-2021, 12:47 AM

Quote:

Originally Posted by engnrsrule

I considered various levels of Solar as a way to extend boondocking. I had bought a small solar battery tender to help keep my Toad battery from going dead while towing. Since adding a battery tender connected to the motor home umbilical, I wondered if the little solar panel would be of any use on the Roadtrek.

On our last trip I boondocked for 6 days. I used coach batteries for fridge controls, lighting, ceiling fan and maybe watching a movie at night. My batteries would be between 85-90% by morning. With the panel on the dash and plugged in to an accessory jack the coach batteries were 100% by evening.

If you are not a heavy power consumer this might be a good option.

https://www.amazon.com/dp/B07DFCDKLJ

That 6w panel brought you to 100% after a day of charging? Surprised it was able to put much back into the batteries.

GeorgeRa · 07-01-2021, 03:20 AM

Quote:

Originally Posted by peteco

That 6w panel brought you to 100% after a day of charging? Surprised it was able to put much back into the batteries.

Possible recharging with 25Ah battery. Just unlikely to use only 10-15% of 25Ah battery for miscellaneous loads and a movie player for an evening and night, that is only 2.5Ah.

engnrsrule · 07-01-2021, 03:49 AM

My coach batteries are 235Ah. My % is from shunt based victron battery monitor. Ceiling fan running all night, fridge and 6 aux computer fans running 24/7. Also have water pump.

GeorgeRa · 07-01-2021, 04:02 AM

Quote:

Originally Posted by engnrsrule

My coach batteries are 235Ah. My % is from shunt based victron battery monitor. Ceiling fan running all night, fridge and 6 aux computer fans running 24/7. Also have water pump.

These numbers don't add-up. 6W solar panel can give you about 0.5A x 5hrs = 2.5Ah of harvesting per day. 10-15% drain per night of your 235Ah battery is 23Ah to 34Ah. Somewhere there is a decimal point error.

engnrsrule · 07-01-2021, 04:32 AM

I was surprised too but I will add that the panel is set up with battery tender circuitry that regulates the current with state of charge and not a flat .5 amps, and last week when observed was pretty long days and bright sunlight.

GeorgeRa · 07-01-2021, 05:22 AM

I = P/V, so 6W/12V = 0.5A, so in best case 6W panel will give you 0.5A for 5hrs average harvesting time per day which is 2.5Ah.

markopolo · 07-01-2021, 01:13 PM

Quote:

Originally Posted by engnrsrule

............... My % is from shunt based victron battery monitor. .....................

There was a topic on the forum not long ago that brought to light that out-of-the-box default settings on the Victron monitor would likely result in batteries being much less than 100% despite the monitor indicating 100%.

markopolo · 07-02-2021, 04:21 PM

Like George, I had to look at the math on this:

235Ah*85%SOC = 200Ah remaining

35Ah*110% (charge inefficiency estimate) = 39Ah to be replaced at approx 14V.

39Ah*14V=546Wh

546Wh / 6W panel = 91 hours of charging needed...............

It seems that it is either that the Victron monitor is not programmed to accurately recognize when the batteries actually are at 100% SOC or alternately, if the Victron monitor is optimally set up, then is it possible that the Victron monitor is fooled by the low current (mA) coupled with high voltage from a direct connect solar panel (no controller)? Would mA current coupled with 15V, 16V, 17V etc. from the small panel create some sort of surface or superficial charge that's enough to fool the Victron monitor?

eric1514 · 07-02-2021, 08:27 PM

The Victron default is 13.2v and 4% tail amps. I think it would be really easy make it read 100% with that small panel.

booster · 07-02-2021, 09:24 PM

Yep more likely the monitor just saying the batteries are full, than actually have it happen with that little solar.

The 6 watt panel will never get to amps high enough not to trigger that part of the charging cycle even unregulated, so all it has to do is get to 13.2v which is a pretty low threshold.

Even the readings at repeated less than full, like the 85-95% are possibly way off if the monitor rarely ever got to even a very low threshold like the defaults. Every cycle will get further off of actual without resets.

engnrsrule · 07-02-2021, 09:53 PM

So I do not think of this as an unregulated solar panel. This panel, billed as a battery maintainer does regulate its output so as not to overcharge. Its performance as a battery maintainer in my TOAD was quite satisfactory, enough to offset battery drain from the ignition being on and supporting the brake buddy.

I am just reporting what I observed. I haven't "done the math" but I did follow steps for the Victron setup and believe it functions properly. Have boondocked for as long as 8 days and watched the monitor closely. When we left it was reading in the 60's percentage wise. Took about 3-5 hours on highway to get back to 100 as I recall.

booster · 07-02-2021, 09:59 PM

Quote:

Originally Posted by engnrsrule

So I do not think of this as an unregulated solar panel. This panel, billed as a battery maintainer does regulate its output so as not to overcharge. Its performance as a battery maintainer in my TOAD was quite satisfactory, enough to offset battery drain from the ignition being on and supporting the brake buddy.

I am just reporting what I observed. I haven't "done the math" but I did follow steps for the Victron setup and believe it functions properly. Have boondocked for as long as 8 days and watched the monitor closely. When we left it was reading in the 60's percentage wise. Took about 3-5 hours on highway to get back to 100 as I recall.

The two important settings in the Victron are the tail/return amps, which if you have AGM will probably be in the .5-1%C range depending on brand and style and the absorption voltage will be in the 14.3v range. From 60% it is unlikely would get to full by those specs in 3-5 hours.

GeorgeRa · 07-02-2021, 10:04 PM

Quote:

Originally Posted by engnrsrule

So I do not think of this as an unregulated solar panel. This panel, billed as a battery maintainer does regulate its output so as not to overcharge. Its performance as a battery maintainer in my TOAD was quite satisfactory, enough to offset battery drain from the ignition being on and supporting the brake buddy.

I am just reporting what I observed. I haven't "done the math" but I did follow steps for the Victron setup and believe it functions properly. Have boondocked for as long as 8 days and watched the monitor closely. When we left it was reading in the 60's percentage wise. Took about 3-5 hours on highway to get back to 100 as I recall.

It seems as your battery is large enough to keep up with your loads for 8 days. Believing that the Victron is working well is great for your piece of mind, math in this case is not necessary.

engnrsrule · 07-02-2021, 10:04 PM

I have 2 6v deep cycle lead acid golf cart batteries in series, 235 Ah.

booster · 07-03-2021, 12:07 AM

Power frugal users with propane frigs can get by on 20ah a day, so that bank would probably be able to do that with the little bit of solar.

eric1514 · 07-03-2021, 01:44 AM

Quote:

Originally Posted by engnrsrule

I have 2 6v deep cycle lead acid golf cart batteries in series, 235 Ah.

Check out this thread

VICTRON MONITOR SETTINGS

I went through the same scenario you're experiencing. The default settings for the Victron monitor are not working for you. I have similar batteries and I had to change my settings from 13.2v to 14.3v and 4% tail amps to .5%. I changed other things like charge efficiency and Peukert, but right off the bat, you should change the "charged voltage" and the "tail current" in your battery settings. I'm not saying my setting are correct for you. Mine favor the PD converter that's in my coach. As it came from the factory, the meter was resetting to 100% well before the batteries were full as evidenced by their voltage and the amount of current they were still accepting.

You're not doing anything wrong and the meter is working correctly it's just that the way the Victron comes from the factory is deceiving you.

markopolo · 07-03-2021, 11:50 AM

Nicely worded post Eric. I was struggling a bit to come up with the right message needed to further encourage engnrsrule to check and adjust those settings and not come across as criticizing the parts or installation. I think your post will accomplish that.

engnrsrule also has a PD charger now after replacing the Tripp-Lite so your settings should work for him.

Adjusting the settings will mean that the monitor most likely won't get to 100% from the small solar panel alone but that's how it should be in this example. This way, he'll have to make sure to periodically get the batteries to actual 100% (not the premature Victron default 100%). Basically this means that he'll have to plug in periodically and let the PD charger do its charge cycle. Driving + solar might also reach the new, more accurate 100% setting.

If one doesn't recharge lead acid batteries to actual 100% (or very near actual 100%) then lead sulfate builds up and hardens causing permanent loss of capacity and premature battery replacement.

booster · 07-03-2021, 12:37 PM

If the setting are on the too conservative side to give 100% full, this is again the same issue we have seen repeated over the last few years. The worst part of it is that there really isn't a good, reasonable cost, fully automatic solution to the problem unless you are capable of building an automated control system your self.

It sounds like these are GC wet cells

Quote:

2 6v deep cycle lead acid golf cart batteries in series, 235 Ah.

so the settings would be different than the more common AGM settings we usually talk about. Absorption could be as high as 14.7v and the tail/return current as high as 3% on them depending on manufacturer, condition, and age. On wet cells is is always a good idea to determine your own tail/return current as it will change over time more than other styles, I think.

The PD charger is one of the few ways to get the batteries 100% full, but will likely take operator intervention to get there because the 4 hour charge cycle would not be long enough for anything less than very shallow discharges. The .5 amp from the the solar probably won't even cover the inefficiencies in charging on 235ah of wet cells so tough for it to top off after driving or a less than full shore charge, I fear.

One of the big problems of using trickle level charges for long term recovery charging is that with a lot of the charge being not very useful as mentioned above, the monitor gets the whole recharge reading very confusing. The Victron doesn't count 10ah any differently if they are put in the battery in 15 minutes or 15 days, but how much is actually stored will likely be considerably different and all the extra time will also allow more life shortening sulfation. Wet cell manufacturers seem to like about 10-12%C charge rate, although up to 20% can be done if the batteries stay cool enough and don't gas too much, at least for a while of the cycle. This can make for a quite long time period being needed to get to 100% full.

On the good side, if these really are GC wet cells, the SOC can be easily checked and put all questions to rest. The OP and the rest of us will know exactly how full the batteries are getting when the Victron says they are at 100%.

markopolo · 07-03-2021, 01:31 PM

14.3V as the charged set point is likely the upper practical limit for an RV equipped with a multi-stage PD charger. Manual intervention to ensure timely completion of charging is an option as Booster points out but if one doesn't want to watch over it then a PD charger will still get a battery bank to 14.3V/14.4V and low tail current because of the interplay of four modes.

14.4V Boost Mode (aka Bulk) transitions to 13.6V Normal Mode which then transitions to 13.2V Storage Mode (aka Float) which then switches to a 15 minute 14.4V Equalize Mode every 20-30 hours and the back to 13.2V Storage Mode.

If a moderately sized battery bank doesn't make it to a within charger capability spec 100% on the run through the first three modes then the automatic 15 minute 14.4V Equalize Mode will take it across the finish line. I'd surmise that it would often happen on the first instance of Equalize mode if not during the prior Boost Mode but if it doesn't then subsequent automatic 15 minute 14.4V Equalize Mode occurrences every 20-30 hours will do it.

Note that this is a within charger capability spec definition of 100% and not necessarily the battery manufacturer definition of 100%. This could be described as doing the best you can with the equipment you have.

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