Hot water with electricity from solar panels

[sailinglegend]

Originally Posted by noelex 77

I don't see any evidence that most people are using charging programs with absorption times that are too short.....but most are between 1.5 to 3 hours as a default.

There's your own evidence!!!!!

Many controllers aren't adjustable, and if they are most owners never adjust them, or if they do they probably don't really understand what they are doing. If it drops down to float quickly they are happy because they think their batteries are fully charged.

[noelex 77]

Originally Posted by sailinglegend

There's your own evidence!!!!!

Many controllers aren't adjustable, and if they are most owners never adjust them, or if they do they probably don't really understand what they are doing. If it drops down to float quickly they are happy because they think their batteries are fully charged.

I disagree with you that the default settings are too short and I would not recommend extending the default time in most instances.
If you have wet cell batteries and check the electrolyte regularly, longer absorption times do little harm, but you need to be very careful with Gel batteries and most AGM batteries.

If you have multiple sources of charging that are regularly utilised such as an external regulator on your alternator, a shore power battery charger, wind, or you use multiple solar controllers, then I would recommend the opposite ie shortening the absorption time from the default if you have Gel or AGM batteries.

The optimium absorption time can be worked out by looking at the battery return amps. My system does this automatically and the typical absorption times are shorter rather than longer than the defaults. Unfortunately, it is different for every cycle, but although it is complex it is possible to manually measure this for a typical cycle.

Try doing this on your own system. You are obviously convinced the default absorption time is too short. I think this measurement would show otherwise.

[Dockhead]

Originally Posted by noelex 77

. . .
If you have wet cell batteries and check the electrolyte regularly, longer absorption times do little harm, but you need to be very careful with Gel batteries and most AGM batteries...

Actually you can corrode the plates with too much absorption time with flooded cells. Overcharging is actually just as harmful as undercharging, with lead acid batteries.

I lost my last set of batteries actually due to overcharging -- long motoring (Kiel Canal) with an Adverc regulator, which cycles high absorption voltage on and off on the theory that it doesn't harm the batts so long as the electroyte doesn't boil.

Not enough absorption or at too low voltage will sulphate the plates, however.

It's important to get this right.

[noelex 77]

Originally Posted by Dockhead

It's important to get this right.

Indeed.

Many more batteries die from undercharging rather than overcharging so in general I am in favour of reasonably aggressive settings at least with wet cells, but it can be overdone. With multiple charge sources the risk is increased.

With more aggressive settings the battery monitors report the battery is accepting more amp hours. This can wrongly encourage the use of things like long absorption times, as has been suggested in the thread.

People think they are squeezing more energy into the batteries unfortunately, the excess goes mainly into heating up the battery and boiling away the electrolyte rather than into increasing the real battery capacity. This can damage Gel and AGM batteries and in extreme cases wet cells as you experienced.

[Waterrat10]

Elements are simply resistors with a certain ohm rating they do not care if AC or DC. Determine your ohms per panel and match number of panels and ohms for optimum efficiency and find element with similar ohms.

I found a few links that some might find helpful.

How to convert AC water heater to DC

The PDF cites some additional sources with good data.

http://waterheatertimer.org/pdf/Phot...ter-Heater.pdf

[Sulaire]

Regarding absorption times, i agree too long in a lab setting is not good, but when you live onboard 24/7, systems are going on and off all day and night, so the batteries are rarely in a lab setting.

Couple this with the fact that when you live on anchor without a genset as we do, the batteries only get daylight hours charging (lets forget wind gen for the moment), then i consider it important to not let your batteries go into float mode too early, and as such I have my absorption time set to 24 hours on my solar controllers.

If you want to use the inverter to heat water as we do, then i think it even more wise to do the above, otherwise your batteries will never achieve full charge.

[mrm]

Originally Posted by Dockhead

I wonder if you could feed them DC power? I don't really see why not.

Sure, usually the first circut in a SMPS is a rectifier anyway..

In that case, you wouldn't need an inverter at all, and a variable power-dump load would be a cinch. You'd need a changeover switch, though.

Yes but..

The controller would be very simple -- just reduce output voltage from max until input voltage reaches the required float voltage, and if that is unachieveable, then zero. Switch it on manually after the charger is in float, or if you can get a signal from the solar controller indicating float status, you can use that to switch on the dump load automatically.

I don't think it will be that easy, rather, some data (signal) on an available power budget made available by the solar/charge controller would be needed. At least to me this looks like a logical place for such signal to be generated, as it is the MPPT which 'knows' the momentary (shading, insolation) capabilities of the panels.

I see a problem with your proposed control algorithm. If the variable power dump starts from max power the charge controller may interpret this as a condition to switch back to bulk.. I think that 'dump enable' and 'dump power adjust' signals should ideally come from the solar charge controller.

[Anders]

Originally Posted by noelex 77

My controller is set up with an advanced system that adjusts the absorption time based on the battery return amps.

What controller do you have?

[Dockhead]

Originally Posted by mrm

Sure, usually the first circut in a SMPS is a rectifier anyway..



Yes but..



I don't think it will be that easy, rather, some data (signal) on an available power budget made available by the solar/charge controller would be needed. At least to me this looks like a logical place for such signal to be generated, as it is the MPPT which 'knows' the momentary (shading, insolation) capabilities of the panels.

I see a problem with your proposed control algorithm. If the variable power dump starts from max power the charge controller may interpret this as a condition to switch back to bulk.. I think that 'dump enable' and 'dump power adjust' signals should ideally come from the solar charge controller.

This all sounds right to me.

My inspiration for this was those clever fridge controllers which freeze down a cold plate when excess power is available judging from voltage.

I guess the flaw in this is that the solar controller might go back into action thinking that charging is needed and start the charge cycle again, which could mischarge the batteries.

But the battery charger on my boat is not fooled by DC loads, even very heavy ones which pull the system voltage down quite far, to flip out of float mode. How does it know the difference? Do solar controllers work the same way?

[sailinglegend]

Originally Posted by noelex 77

....The optimium absorption time can be worked out by looking at the battery return amps......Try doing this on your own system.....

I've been doing this for 12 years as a Liveaboard, and my AGMs have lasted 12 years. The absorption time is set to 8 hours for my 1050 Ah bank, but they still drop down to Float too early with my Victron charger. It is programmed to stay on absorption for 5 times the time it takes to get to the absorption voltage. So if it takes 1 hour to get to absorption it stays there for 5 hours. If the bank is fairly well charged and it takes 12 minutes to get to absorption it stays there for 60 minutes.

This indeed is a complex subject, but the main point I'm try to get across is that Float doesn't mean your batteries are 100% charged, so don't put on a load that could stop Float mode still charging your batteries.

By all means add you iPhone or computer chargers but keep an eye on the charge voltage. If that drops below the Float voltage then your finishing charge may never finish. Chargers should not switch back to Absorption voltage if this happens because they have a delay built in. You don't want them to repeatedly keep switching back to absorption voltages for another 3 hours.

My simply message is:
Float doesn't me your batteries are 100% charged.

[StuM]

Originally Posted by Anders

What controller do you have?

You can do that with an Outback Flexmax.

[sv.antea]

One thing would worry me if you use DC for the water heater. The thermostatic switch that turns the element on and off is rated for AC current. When it opens, it arcs, but the arc is extinguished as the voltage drops to zero on the next current direction reversal. DC current would keep the arc going for longer time, possibly damaging the switch. I am sure there are ways around it though.
I have thought of dumping the excess solar into water heater in conjunction with keeping LiFePO4 battery happy and came across another complication. When the thermostat turns the heater element off, care must be taken that the charging of batteries, presumably already at desired SOC, doesn't resume. There must be, particularly with LiFePO4, another system for stopping the charging.

[Ironman162]

The thermostat drives an auxiliary relay with several contacts in series to switch the dc current is the one on my boat.
Also the wind turbine use it for a dumping load directly so that it bypasses the thermostat.
It has worked well for many years without failing.

Switching dc is a well known method and no need to re invent it all again.

[Dockhead]

Originally Posted by sailinglegend

. . .

This indeed is a complex subject, but the main point I'm try to get across is that Float doesn't mean your batteries are 100% charged, so don't put on a load that could stop Float mode still charging your batteries.

. . ..

This raises an interesting question -- what kind of load does it take to flip the charger back into absorption?

My Victron Multiplus does not seem to be very eager to do this. Even very heavy loads like my 3kW inverter, do not interrupt the float phase, as long as they don't go on long enough to significantly discharge the battery.

Is that just the way it is programmed? Are other chargers different? I would think that it would be a significant flaw in the charging algorithm, which could damage batteries, if absorption phase gets repeated without a corresponding depth of discharge occurring first.