Dumping excess solar energy

If the charging regime goes PV->LiFe->AGM then you are losing a chunk of energy continuously through the B2B charger and it's inefficiency. I don't know how significant that is.

I was suggesting a dual-bank charger that charges both batteries directly. You'd then need some way of switching in the B2B charger, but I don't know the best way of doing that.

It's all about simplicity vs efficiency...
 
Don't some charge controllers have a dump load facility? I'm sure I've heard of people using this to drive things like fridges.
What if we set up the controller to activate the dump load once the main bank reaches absorption... and the dump load itself is just the LiFe bank... which is permanently hooked up to the B2B (other than when it disconnects itself due to reaching minimum voltage).

I'm starting to think that there could be a (small) market for self contained batteries that contain a mixture of lead-acid and lithium cells, containing the B2B etc... you'd gain the longevity and charging efficiency of LiFe, but without all the cost.
 
Unfortunately my Victron doesn’t as such. It does however have a load output that I can configure to come on when the battery reaches a particular voltage. Alternatively the battery monitor can switch a relay at a particular state of charge or voltage. Might be able to do something with either of those, but it runs the risk of diverting some charging capacity into the load rather than the battery.
 
Thanks to everyone for their thoughts and comments so far, it’s been interesting, Now, assuming I could find a maximum £250, that would give me two options:

1. Buy a small LifePo4(20Ah) and B2B charger for the Solar->LifePo4->AGM charging suggestion. This would also increase useable battery capacity by ~30%

2. Buy another 80W panel and MPPT controller. Caveat: Would need to remodel the coachroof and the panel would be shaded by boom & mast during the most productive sun hours when on my mooring.

A third, negligible cost, option would be to fit a couple of USB charging sockets for small battery packs/devices that are only energised when certain battery/charging criteria are met with my existing system.

All options have their own inefficiencies, but are possibly better than the current situation to some degree. Doing nothing is not an option.
 
Where are you putting your panels at the moment? If the space is not already in use, have you considered hanging them on the guard-wires? I've got a 100w panel on one side, might add one on the other side at some point. Used cheap clamp fittings and just used light cord to hold it up at the required angle (I have an antenna pole in a handy place).
In your profile pic I can see dodgers with nothing covering them so I'm guessing this space is available?
 
On my (tidal, muddy-ditch bank) mooring, the boat is tied up north-south. Port side would almost never get any useable sun and be easily damaged, starboard would maybe be a little less problematic, Unfortunately, anything not very firmly fixed down round here is likely to disappear! Part of my power usage when not aboard is for a remote camera and auto-dialling alarm ?
 
I might not be fully understanding how your boat is moored but the side of the boat shouldn't make much difference- the idea is that the panels are swung to the horizontal, like wings, and you only drop them for passage making in lively conditions.
I also hadn't fully appreciated that you need more power for when the boat is not actually in use. Most people just worry about self-discharge and a tiny panel will easily keep up with that.
 
I'm dazzled by all the deep thinking, brilliant ideas suggested.

A very simple solution would be to use a low power 220v inverter possibly switched to the "load" output with a voltage sensitive relay to charge the numerous 220v widgets on board. All my such widgets (rechargeable lamps, shaver, hand-held etc) rely on switching on the main 1000w inverter which makes appreciable demands on the battery even when the output is not loaded.
 
The main concern is that anything easily removed, will probably disappear within a season. I’ve previously lost an anchor, an outboard, jerrycans of fuel and a portable heater on separate occasions. In a different location ‘wings’ would be worth considering.
Currently I have a 40w panel on the forward hatch and another just aft of the cockpit.
 
A panel aimed to catch evening or morning sun may be worthwhile.
An extra hour of charging time can be more valuable than extra amps of peak power.

It might be useful to log actual currents and voltages at the battery over a few days? Also voltage at the panel, to get some indication of how many hours you really do have unused solar energy avaiable.
 
I replaced everything a few years ago. Process was as follows.

Do an energy budget. It was clear to me that the fridge was the major consumer so I bought a WattMeter and measured consumption over 24 hours on various thermostat settings. Everything else I estimated based on rated consumption and two scenarios: a day sat at anchor and a day sailing with all instruments and auto-pilot. Motoring was discounted as anything over an hour generates a large amount of energy.

All up I need around 750Wh a day.

I decided to go lead acid as lithium was still extortionate and I wanted to survive 3 days with no sun and no engine use. This meant I needed a battery bank that could deliver 2250Wh without discharging excessively. That makes about 190Ah. So I approximately doubled that and bought 370Ah of batteries.

The solar array therefore needed to produce 750Wh reliably every day. After looking up insolation values for Istria from April to September and allowing for horizontal panels and conversion values I landed at around 300W of solar.

It all works fantastically, batteries are normally fully recharged by lunchtime and the solar then runs everything until dusk when the batteries take over again.

On normal sunny days at anchor, it generates between 900Wh to 1kWh.

Previously I had only 120Ah of battery capacity and the battery never lasted very long due to the depth of discharge it experienced.

Screenshot_20200717-110007.jpg

On the graphs, white is Bulk charging, light grey is Absorption and dark grey is Float. This was the last 4 days, the lowest day was a mix of motoring and sailing so most of the energy came from the alternator and the solar then just shuts down. Last 2 days were at anchor, and the 960Wh day was a long day sail, mostly on autopilot.

Max and min battery voltage are the highest and lowest battery voltage seen by the solar charger over the 24 hour period.
 
Mine is just white most days at the moment. Hoping to see some light or even ?some dark grey today! Last dark grey was 12 days ago. Hence the thread. Can’t reduce consumption any more, more solar is complicated and more lead acid won’t help... hence the thread. ?
 
MarkCX said:
All data is currently logged to a Raspberry Pi at 5 minute intervals. Probably too much data ?
Click to expand...
So how many hours do you actually have excess solar capacity?
How do you see that looking when it's mot midusmmer?

How many hours is the solar actually in bulk mode?
What is your peak wattage delivered to the battery and how does that compare with the nominal power of the panel(s) allowing for angles of panels?

I suspect the charge controller is not making the best of the available solar power.
 
MarkCX said:
Mine is just white most days at the moment. Hoping to see some light or even ?some dark grey today! Last dark grey was 12 days ago. Hence the thread. Can’t reduce consumption any more, more solar is complicated and more lead acid won’t help... hence the thread. ?
Click to expand...
Unless your bulk current limit is set to a low value, it's only in the grey areas you have any excess power to charge a lithium battery with.
 
You must log in or register to reply here.

Other threads that may be of interest

Baggywrinkle
Test run of the Lithium system that has been sitting in my cellar for most of this year.
Replies
14
Views
511
Baggywrinkle
Baggywrinkle
geem
First 6 months with no gas onboard
2 3
Replies
41
Views
1K
Baggywrinkle
Baggywrinkle

Share this page

Top