Dumping excess solar energy

[MarkCX]

I’ve got 80W of solar panels charging a 125Ah AGM and a smaller AGM starter battery with a VSR via a Victron SmartSolar MPPT controller. As charge acceptance decreases, I’m wasting potential solar energy.

Just wondering, is there a relatively inexpensive way to ensure that the AGMs get absolute charging priority, dumping any excess charging capacity into a portable lithium power pack?

 

[Kinsale373]

Do you need the additional energy ? If you really do then add a Battery Capacity to store it. If not why spend money on something you don't need. Sunlight is free so you're not wasting any resourse.

Kinsale 373

 

[RJJ]

I’ve got 80W of solar panels charging a 125Ah AGM and a smaller AGM starter battery with a VSR via a Victron SmartSolar MPPT controller. As charge acceptance decreases, I’m wasting potential solar energy.

Just wondering, is there a relatively inexpensive way to ensure that the AGMs get absolute charging priority, dumping any excess charging capacity into a portable lithium power pack?

Caveat - not an expert, and quite idle.

I think the typical way is to plug in, directly to the regulator, something like a fridge that can take a wide range of charge inputs (i.e. less sensitive to whether it's 12.5 or 13.3 or 14.8 volts). My regulator has a dump output for that purpose, which works well if you're living on board, which I haven't fitted (see para 1).

Depending on the "dump" output from your regulator (look it up), I can't see why you couldn't charge a battery from it but if that battery's input voltage is sensitive presumably you'd need to regulate it again?? Sounds a PITA and also electrically inefficient. Why not just charge your portable battery from the cigarette lighter and, if you're going to spend money anywhere, buy an extra 12v battery?

 

[MarkCX]

Maybe I’ve just got too much time on my hands to think about stuff (and maybe over think it)!

Sunlight is free, but it is a scarce resource, so not making the most of it is letting it go to waste. And I did say inexpensive.

Most days I don’t get enough sun to fully charge the batteries I have, but occasionally I do. A larger lead acid battery wouldn’t help. But a lithium power pack (which I have) will be relatively happy about partial charging and being left partially charged, will accept a steady current right the way up till full, and will be useful for running various usb devices when it does have some charge in it.

 

[MarkCX]

My regulator has a dump output for that purpose,

Don’t think mine has that facility, but interesting and worth a look in the manual. I know it has a load output, but I don’t think it can be configured in that way.

With regard to efficiency, almost any loss is better than total failure to make use of it.

 

[fredrussell]

My mate has just installed a means of heating water in his calorifier with ‘surplus’ solar output.

 

[MarkCX]

Similar sort of idea. It’s just trying to figure out how to get that power while squirting as much juice as possible into my batteries. For example, at the moment my panels can provide 5 amps of current, my batteries are accepting 1 amp right now as they are almost ‘full’. I want that extra few amps to be doing something useful!

 

[RJJ]

Maybe I’ve just got too much time on my hands to think about stuff (and maybe over think it)!

Sunlight is free, but it is a scarce resource, so not making the most of it is letting it go to waste. And I did say inexpensive.

Most days I don’t get enough sun to fully charge the batteries I have, but occasionally I do. A larger lead acid battery wouldn’t help. But a lithium power pack (which I have) will be relatively happy about partial charging and being left partially charged, will accept a steady current right the way up till full, and will be useful for running various usb devices when it does have some charge in it.

And can you charge it from the cigarette lighter?

 

[MarkCX]

Yes. I can do that, and that is what I am doing at the moment to some degree. That does mean that I need to be around and paying attention which is a bit of a faff and also does not ensure that my AGMs are getting absolute priority.
The point of me asking was to find some way to automatically make use of excess charging capacity without slowing down the charging of the boat’s main batteries in any way. It might very well not be possible, which is fine, but if it is then I’d like to know about it.
Ideally I’d like a socket that is only ‘on’ when I have excess power available.

 

[HissyFit]

The hardcore off-grid solar community are eagerly awaiting the next generation of supercapacitors to arrive, but they have been doing so for some time.

The benefits of supercapacitors as they currently stand are:
Rapid charging
Massive charge capacity (4 or 5 Farad is common)
Can deliver power as a significant burst
Million + lifecycles
No danger of overcharging

The downsides are:
Low voltage across individual cells
Voltage drop-off is linear
Lack of developed applications and related circuits.

Given the inconveniently long time required to recharge a flat starter battery, and the lower power delivery of batteries intended for use in house banks, it would be wise to carry a supercapacitor jump starter on board. This is one area in which the technology is current and mature. I'd go as far as saying that it should be possible to make a specific cranking battery redundant, allowing the purchase of an extra house battery, thus increasing the house bank's charge acceptance rate.

If the application circuits were available, supercapacitors would be ideal to dump excess charge into, thus enabling battery charging passed the usual end of useable daylight.

 

[TernVI]

The hardcore off-grid solar community are eagerly awaiting the next generation of supercapacitors to arrive, but they have been doing so for some time.

The benefits of supercapacitors as they currently stand are:
Rapid charging
Massive charge capacity (4 or 5 Farad is common)
Can deliver power as a significant burst
Million + lifecycles
No danger of overcharging

The downsides are:
Low voltage across individual cells
Voltage drop-off is linear
Lack of developed applications and related circuits.

Given the inconveniently long time required to recharge a flat starter battery, and the lower power delivery of batteries intended for use in house banks, it would be wise to carry a supercapacitor jump starter on board. This is one area in which the technology is current and mature. I'd go as far as saying that it should be possible to make a specific cranking battery redundant, allowing the purchase of an extra house battery, thus increasing the house bank's charge acceptance rate.

If the application circuits were available, supercapacitors would be ideal to dump excess charge into, thus enabling battery charging passed the usual end of useable daylight.

4 or 5 farads is not a massive charge capacity. A farad is a coulomb per volt. A coulomb is an amp times a second. So if you can charge your farad to a lot of volts it might be useful.
The energy stored is 1/2 CV^2, so to get some watt hours, you need a lot of Vsquared.

What you can do, with the right precautions, is to raise the charge voltage and hence current, taking care that your batteries don't overheat.

 

[Alan S]

It can take more than 100 Amps for for a few seconds to start a diesel engine, several hundred times the capacity of the supercapacitors you mentioned.

I Farad is 1 Amp.second.

 

[MarkCX]

Interesting tangent, however...
Just learned that wind and water turbines frequently (always?) use dump loads to prevent overcharging and/or damage to turbines. I basically want to do a similar thing. My batteries are relatively new and are the largest I can afford & have space for. Changing or adding to them is not an option.

 

[Alan S]

One possibility is a pwm solar charge controller that dumps excess current into a useful load like a peltier fridge. Not sure if there are any commercially avilable so I made one (fridge and controller) using a pic microcontroller. It does work but my 40 W panel is not really enough. I could give you more details if you like.

 

[KompetentKrew]

Yes. I can do that, and that is what I am doing at the moment to some degree. That does mean that I need to be around and paying attention which is a bit of a faff and also does not ensure that my AGMs are getting absolute priority.
The point of me asking was to find some way to automatically make use of excess charging capacity without slowing down the charging of the boat’s main batteries in any way. …

I hope you won't be offended if I say I think you're overthinking this.

The answer depends on your usage - how often you're on the boat and what you're using power for, but if it's important enough then the answer is to expand or replace your main bank.

I get the impression that such an expense wouldn't be justified, in which case you'll either be off the boat long enough for the lithium auxiliary to fully change, or you'll be on the boat and can unplug it when your main battery is low.

I suppose you want all power going to the AGMs when their voltage is low enough to accept bulk charge, so you could switch the lithium's charger on and off depending on the voltage measured on the main battery. Then what do you run off the lithium battery?

I now have four of these large power banks - being USB-C I can run my MacBook off them; for that purpose one lasts 3 or 4 hours. I charge them when on shore-power or on sunny afternoons at anchor.

 

[geem]

I’ve got 80W of solar panels charging a 125Ah AGM and a smaller AGM starter battery with a VSR via a Victron SmartSolar MPPT controller. As charge acceptance decreases, I’m wasting potential solar energy.

Just wondering, is there a relatively inexpensive way to ensure that the AGMs get absolute charging priority, dumping any excess charging capacity into a portable lithium power pack?

With all due respect, thats a very small amount of solar. I have 720w charging my domestic bank. When there is plenty of sun, i heat water with the spare capacity
What ever you might do is likely to need some financial investment but with your tiny amount of solar the benefits would appear to me minuscule.

 

[Baggywrinkle]

I have 300W charging a 370Ah domestic bank in the med, just right IMO for a 11,5m sailing boat where the biggest load is a fridge. I have never been below 90% charge - usually 94-95% when I wake up in the morning. When sailing, the solar is enough to run all instruments and the autopilot.

I have a Victron MPPT 100/ 30 and it simply supplies current on demand when the batteries are full or even when charging. Just use electricity as needed and the solar output jumps to compensate as the battery voltage drops. The batteries then hover around 99-100%. No need for any additional circuits.

To get solar hot water, direct heating in blacked pipes or a shower bag works a treat.

Don't overthink everything IMO .... just size the solar and house bank as appropriate for your power needs and location and let it get on with it.

 

[HissyFit]

4 or 5 farads is not a massive charge capacity. A farad is a coulomb per volt. A coulomb is an amp times a second. So if you can charge your farad to a lot of volts it might be useful.
The energy stored is 1/2 CV^2, so to get some watt hours, you need a lot of Vsquared.

I see I've gone off at too great a tangent here. (That's me) I only see the outline of an application, but electronics is not really my thing. Individual cells may be low voltage, and you lose capacity by wiring them into a nominal 12v stack. It would take many MOSFETs to switch them from parallel to serial, like in a switch mode power supply, thus preserving the rapid charge/discharge capability whilst overcoming the low voltage.

What you can do, with the right precautions, is to raise the charge voltage and hence current, taking care that your batteries don't overheat.

This is standard operation for intelligent chargers as they stand today. I goes without saying that you should get the best use out of what you already have before looking to upgrade.

 

[HissyFit]

I Farad is 1 Amp.second.

1 Amp.Second/Volt

I wonder how those jump-starters are so compact. Maybe they rely on being only supplemental. Anyway, my introducing immature tech is not helpful to the thread.

 

[MarkCX]

Yup. Definitely overthinking things. Can’t go to work at the moment. Spending a lot of time staying on the boat, but can’t go anywhere as engine is currently out of service!
It’s a small 23’ boat so space is at a premium, hence limited solar capacity. Same applies for the battery - it’s the biggest I could fit in the space available. With the changeable weather and my (albeit frugal) power demands, not seen 100% for over a week. I have no shore power.
Anyway, thanks for all your comments & suggestions, all appreciated. I’ll go and think of something else to worry about ?