Old Harry
Well-Known Member
An excellent device, FOR RUNNING DOWN BATTERIES
Dumping excess solar energy
- Thread starter MarkCX
- Start date
MarkCX
Well-Known Member
Yes, but not bad for a dump load. Nice sunny day, usually means warm, so cool drink welcome. I don’t have any boat fridge at the moment.
TernVI
Well-Known Member
For people with not enough to do, who worry too much about not reaching 100% on their batteries, maybe there's a market for a small exercise bike with a dynamo?
GHA
Well-Known Member
Depends on the size of your wallet. Not getting lead acid batteries fully charged regularly is a guaranteed way to kill them.
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MarkCX
Well-Known Member
Now I have a lot more time than money ?
Boater Sam
Well-Known Member
I have put an advance order in for a Flux Capacitor.
Sam.
Sam.
Kelpie
Well-Known Member
Interesting thread. I had incorrectly assumed that you needed to throw all of your charging capacity at a lead acid bank until it was full, but that the last wee bit would take ages. It hadn't really occurred to me that the diminishing charge acceptance meant there was more and more power available for other things.
Presumably what you run out of is time, rather than actual power?
Thinking out loud, but could you charge a lithium battery alongside the main bank using the excess power, and then use B2B charging to from the lithium to carry on topping up the lead acids after the sun has gone down?
Probably a massive over-complication, I know, but I have been looking into the pros and cons of lead-acid vs LiFePO4 for a while now and lithium is hugely tempting, but also still very pricey. I really like the idea of not putting all my eggs in one basket and having a single ~100Ah lithium battery to supplement a moderately sizes conventional bank.
Presumably what you run out of is time, rather than actual power?
Thinking out loud, but could you charge a lithium battery alongside the main bank using the excess power, and then use B2B charging to from the lithium to carry on topping up the lead acids after the sun has gone down?
Probably a massive over-complication, I know, but I have been looking into the pros and cons of lead-acid vs LiFePO4 for a while now and lithium is hugely tempting, but also still very pricey. I really like the idea of not putting all my eggs in one basket and having a single ~100Ah lithium battery to supplement a moderately sizes conventional bank.
GHA
Well-Known Member
I suspect what a great many boats run run out of is knowledge of how lead acid batteries work....
Then people just leave a solar regulator at default settings which means it goes to float ages before the batteries are close to full and as the little green light comes on showing float that is taken as a full battery. But yes, takes ages on the majority of boats to fully charge a lead acid battery from solar even staying at bulk voltage with all the other loads like fridges switching on and off and shadows/clouds on the panels.
That's where I'm slowly heading, rewiring some battery stuff today which will have extra connections on a bus bar to get a LifePo4 in the system as well. But wiring and connections will end up a bit complex and will need manual help - probably just use a buck/boost converter to do the transfer. Hopefully as well as using excess power it will allow the alternator to be used much better rather than running for hours dripping in the lead acid, LiFePo4 will happily accept a load of current.
Might end up with 2 1/2/b switches!!! ? That will really get the grumpy solent ex schoolteacher types going
Pete7
Well-Known Member
Actually I don't think you have enough solar if most days you aren't able to fully charge the batteries. So I wouldn't worry about the odd really sunny day and a few wiggy amps going to waste, but increase the solar so you can achieve a full charge much more frequently and check the MPPT settings with the advanced options to match the batteries.
If the batteries achieve full charge by mid afternoon and are held there in float, any excess charge will presumably be used on board with other electrical appliances, lights, radio or laptop etc. We are talking about 4A in strong sunshine. This delays the time you start to draw power from the batteries, reducing the overall discharge, which is good and enables the solar to charge up quicker the following day. So I don't think you are wasting those electrons.
Pete
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MarkCX
Well-Known Member
Now that seems to be a possible solution Kelpie. Although possibly (too) expensive, it does turn the thing on its head! I’m not ready to fully shift to lithium yet, maybe when my current (new) batteries die. I wouldn’t even need that large a lithium battery, just enough for a day’s solar.
This kind of thinking is what I like about the forum. Although I’m sure someone will come along with reasons why it won’t work?
This kind of thinking is what I like about the forum. Although I’m sure someone will come along with reasons why it won’t work?
MarkCX
Well-Known Member
The problem is space. 2x40W panels were tricky enough to squeeze on without putting them somewhere with a lot of shading and requiring additional structure to be added to the boat. I reckon I can lose up to 50% of the potential energy due to diminishing charge acceptance. On 2 near identical days I can get 300+Wh if the battery is quite discharged at the start of the day, ~150Wh if already at 90% plus.
Increased solar would be a relatively cheap option if I had anywhere to put it.
Pete7
Well-Known Member
Well, without changing the solar set up and you ending up looking like a solar farm, perhaps there is a another simple cheat.
If by 2pm your batteries are nearly full and they are in float, turn the fridge thermometer down and super cool the fridge and contents until about dinner time when you revert it back to the normal setting. This will delay the time that the fridge then has to switch on in the evening by some hours and reduce the discharge on the house bank. Since you are likely going to be in and out of the fridge making dinner, this is likely when the temperature will rise the quickest inside the fridge, so could be a good gain. Also much simpler than having more batteries to store energy, this way you are using the spare energy straight away to make savings later on. You just need an alarm clock to remind you to change it back at dinner time, or the milk for your Frosties in the morning will be erm, frostie
MarkCX
Well-Known Member
Good idea, I might just get a fridge ?
But seriously, that’s the right kind of idea, making the most of available amps.
But seriously, that’s the right kind of idea, making the most of available amps.
TernVI
Well-Known Member
Your battery capacity needs to be based on what you need to take out.
Being able to store whole day's solar output is one thing, being able to run your lights/fridge/whatever for as long as you want is another.
You might have a goal of being able to spend a long weekend at anchor, using fridge, lights, eberspacher... when the solar is not delivering much.
You might have greater of lesser goals.
Personally I take the view that the name of the game is to reduce engine use for charging to some sort of reasonable minimum. If I have to run the engine once on my yearly long cruise jsut to charge the batteries, I can live with that, doing so ever week, I cannot. There is a diminishing returns thing going on where the gains from bigger batteries, more solar etc deliver less and less actual services on the boat and reduce engine running less.
Using lithium battereis to increase the amount of time your lead acis spend 100% charged is an interesting concept, but evaluating the true cost/benefit of it would be a challenge I think?
The true reduction in life of a lead acid from being left at 90% overnight is probably unknown.
RupertW
Well-Known Member
That’s exactly what I‘m doing right now. Or to be more accurate I have switches and VSRs and an invertor and transformer and cables and connectors and fuses in front of me on the saloon table, and then distracted myself by seeing if there were any interesting arguments on the forum.
MarkCX
Well-Known Member
I’m really careful with how much I use. I doubt it it’s been more than 15Ah a day while I’ve just been sat here on my mooring. Charging iPad and phone, LED lights, Bluetooth amp & speakers, mobile internet router and occasional diesel heater use when it gets a bit chilly. It doesn’t take more than a couple of dull days to get to a point where it takes a week or so to get back to fully charged though.
GHA
Well-Known Member
That significant damage is done by never or very rarely getting fully charged isn't a contentious view in the slightest, as a well respected US marine engineer has said - boat batteries don't die...they are murdered..
MarkCX
Well-Known Member
As a compromise, I like to see the battery fully charged at least once a week, but it’s currently getting on for 10 days ?
Kelpie
Well-Known Member
I've been thinking a bit more about my idea, I don't know enough about these things to work out all the details yet but it may be worth exploring.
My assumptions are:
Lead-acid is badly damaged by not being fully charged every few days/cycles.
Lead-acid becomes less efficient and slower to charge as it reaches full charge- over about 90% becomes painfully slow.
LiFePO4 accepts higher charging currents and remains very efficient to charge right up until it is full.
B2B charging is not completely efficient, but is around 85%.
So to recap, the idea is to use solar to bulk charge the lead-acids, then when they reach around 80-90% SOC start charging the smaller Li battery, which should reach full charge pretty quickly- much more quickly than it would take to top off the lead-acids.
The B2B charger then slowly tops up the lead-acids over several hours, so in the morning you wake up to a brimming full lead-acid bank and a partially depleted Li battery.
Whether all of this is worth doing comes down to the various efficiencies involved, which is where my understanding of things runs out. If you only need to use B2B to top up the last 10%, the Li battery can be quite small- just 10% of the main bank plus a margin to cover the charging losses and a bit of reserve to avoid total depletion.
My assumptions are:
Lead-acid is badly damaged by not being fully charged every few days/cycles.
Lead-acid becomes less efficient and slower to charge as it reaches full charge- over about 90% becomes painfully slow.
LiFePO4 accepts higher charging currents and remains very efficient to charge right up until it is full.
B2B charging is not completely efficient, but is around 85%.
So to recap, the idea is to use solar to bulk charge the lead-acids, then when they reach around 80-90% SOC start charging the smaller Li battery, which should reach full charge pretty quickly- much more quickly than it would take to top off the lead-acids.
The B2B charger then slowly tops up the lead-acids over several hours, so in the morning you wake up to a brimming full lead-acid bank and a partially depleted Li battery.
Whether all of this is worth doing comes down to the various efficiencies involved, which is where my understanding of things runs out. If you only need to use B2B to top up the last 10%, the Li battery can be quite small- just 10% of the main bank plus a margin to cover the charging losses and a bit of reserve to avoid total depletion.
Pete7
Well-Known Member
Sorry, but that violates the first law of boat maintenance, "Keep it Simple".
If you do away with the FLAs for the house bank, have one small engine start battery and a medium sized LifePO4 house battery to run 20-90%, charged by a B2B from the engine battery, I think you would have a simpler system.
But you could buy an awful lot of batteries for the price of the LifePO4.
If you do away with the FLAs for the house bank, have one small engine start battery and a medium sized LifePO4 house battery to run 20-90%, charged by a B2B from the engine battery, I think you would have a simpler system.
But you could buy an awful lot of batteries for the price of the LifePO4.
