Better way of charging batteries?

[zboss]

I was speculating about something...

What if you were to charge a bank of marine capacitors first (such as this one) using your usual charging systems and then have that capacitor charge the batteries.

Its my understanding that the capacitor takes energy like a lithium battery - pretty much as fast as you can give it - and doesn't stop taking the current until it is full.

So, in situations where there is intermittent sun/shade on a solar panel and where the batteries are reaching capacity, the capacitor would take all the power the panel can give in short pulses vs. a lot of wasted current trying to charge the battery directly. Then the capacitor can "echocharge" the battery and the battery can charge at its peak efficiency 100% of the time even if the solar is not optimum.

For example... later in the solar day the batteries are 90% full. In order for them to get to 100% charge it would take hours of trickle charging at low amps but the sun is only going to be up another hour. However, in that hour, the capacitor can take 100% of the energy the solar can give it, while the battery can only accept a fraction of that. Then, the capacitor continues to charge the battery after the sun goes down, resulting (theoretically) in a 100% charged battery.

What do you think? Nuts I am sure.

[Wotname]

Hmm... cost would stop me!

[travellerw]

There are quite a few technical reasons why this wouldn't work well. One being that to use up all the energy in a capacitor you must drain it completely. So if you have a 4-14V capacitor you would only ever use a very small portion of the capacitor. Once the battery and capacitor matched voltage, the current draw would drop off to zero.

For this to work, you would need a HUGE capacitor. Like, 1/2 the size of your boat huge.

[nimblemotors]

Large capacitors are very expensive, like 10x more than batteries.
Instead, you would just get faster charging Lithium batteries for less cost.

[Cottontop]

Originally Posted by travellerw

For this to work, you would need a HUGE capacitor. Like, 1/2 the size of your boat huge.

Yeah, for $1,000 you get 35 kj, which is about 10 wh, which is less than 1 amp-hour at 12 volts (if I got the conversions right).

[sailorboy1]

even if it were possible and controllable, you buy a lot diesel for the cost of that capacitor

[davefrediam]

Other than the issues mentioned above capacitors are only capable of holding their charge for a short time (minutes at most) as they leak like a sieve, hence batteries are used which don't leak as they don't actually store the electricity but chemically convert it in a reversible reaction.

[chris95040]

Originally Posted by davefrediam

Other than the issues mentioned above capacitors are only capable of holding their charge for a short time (minutes at most) as they leak like a sieve, hence batteries are used which don't leak as they don't actually store the electricity but chemically convert it in a reversible reaction.

I think that's what the OP was getting at - a coupling of the "quick charge" capability of a capacitor with the "long term storage" capability of a battery.

Sounds like its not really plausible given some practical matters mentioned here, but its definitely a clever thought.

[jkindredpdx]

Yeah, I say good on ya for thinking and putting your idea out there. Even if it isn't practical today perhaps it will spur some other innovation...

[Maine Sail]

Originally Posted by zboss

For example... later in the solar day the batteries are 90% full. In order for them to get to 100% charge it would take hours of trickle charging at low amps but the sun is only going to be up another hour. However, in that hour, the capacitor can take 100% of the energy the solar can give it, while the battery can only accept a fraction of that. Then, the capacitor continues to charge the battery after the sun goes down, resulting (theoretically) in a 100% charged battery.

What do you think? Nuts I am sure.

The point at which batteries hit absorption voltage, and current becomes limited, is directly proportional to the amount of charge current you have.

With most on-board solar systems they spend the vast majority of their time in bulk because we have low charge current in relation to a very large bank.

For a 400Ah flooded bank it may hit absorption voltage at 75% SOC with a .25C (25% or 100A) charge rate. From the time it hits absorption or "voltage limiting" current begins diminishing thus making the use of the large charge source less efficient.

That same bank won't hit absorption voltage until the upper to high 90% range with a .025C charge rate (0.025% or a 10A solar array). Bulk is the most efficient stage of charging and solar systems on boats are naturally very efficient because of the low charge rate. With most installed solar systems you be talking about trying to maximize approx the last 1% - 6% at best....

[zboss]

Its my understanding, and yes I get we are talking about technology 20 years out, that super capacitors are being developed for use in cars that have 30 or 40% of the charge capacity as lithiums and can slowly discharge their charge.

What about using a small lithium battery in the same way? Have your solar charge your lithium bank and then have the lithium echo charge the lead acid. You may only be talking about the last 1 to 6% but that's OK... you then get longer lasting batteries with lithium performance without the price of a bank of lithium batteries....

So - if your battery bank is 500 aH, you theoretically only need a 50 aH lithium (last 10% of lead acid capacity).

again, don't shoot...

[u4ea32]

Solar and lead acid is a good mix. Generators and LiPO is a good mix.

Solar and LiPO is a waste of money. Generators and lead acid is a waste of money.

A solar + lead system is much, much cheaper than a generator and LiPO solution. About an order of magnitude, but you need enough solar panels, and space for them, which is usually the problem on most boats.

[Tomaz473]

Let us assume a 500 Ah 12V bat. bank.
If you want to solve last 10 % of charging you need 50 AH for this..
The smaller KBI booster stores 35 kJ = less then 1 Ah at 12V.
So you need more then 50 of them.
1) it costs more then 50000 $
2) it weights 1500 LBS
3) it takes all the space you have
All this under assumption you have a DC to DC converter able to charge your batteries from any voltage 4V to 16V at 0% loss - a device like this does not exist. In real life you would need more KBI devices.

It is a lot easier and a lot cheaper and takes a lot less space to add one 100 Ah battery to your bank (50 Ah of useful capacity).
You will discharge the bank less (you now have 600 Ah)
You will be able to charge it faster (you have one more bat. in parallel)
You need no investment in any new electronic - just two cables to connect new battery to existing bank.
Adding one battery is more then 100 times less investment for better result - to me an easy decision.

[zboss]

Originally Posted by Tomaz423

Let us assume a 500 Ah 12V bat. bank.
If you want to solve last 10 % of charging you need 50 AH for this..
The smaller KBI booster stores 35 kJ = less then 1 Ah at 12V.
So you need more then 50 of them.
1) it costs more then 50000 $
2) it weights 1500 LBS
3) it takes all the space you have
All this under assumption you have a DC to DC converter able to charge your batteries from any voltage 4V to 16V at 0% loss - a device like this does not exist. In real life you would need more KBI devices.

It is a lot easier and a lot cheaper and takes a lot less space to add one 100 Ah battery to your bank (50 Ah of useful capacity).
You will discharge the bank less (you now have 600 Ah)
You will be able to charge it faster (you have one more bat. in parallel)
You need no investment in any new electronic - just two cables to connect new battery to existing bank.
Adding one battery is more then 100 times less investment for better result - to me an easy decision.

Yea, I changed it from a capacitor to a lithium battery. Also mentioned that capacitors are coming down the road that store 30 to 40% of what a lithium does now - a long way a way - but still. This is just a theoretical discussion.