My Micro-Hydro system

[FilterGuy]

I have an off-grid mountain cabin being powered by a micro-hydros system.

• The turbine is called a Watter Buddy. https://microhydropower.com/our-products/watter-buddy/ I get a continuous 120 Watt at nominal 24V out of this little turbine (2880 Whr/Day). (I estimate I am using 1300 W-hr/day so that is enough to power the cabin.
• I am currently using 4 6V 230AH flooded lead acid batteries in a 24volt configuration. This is probably more storage than is really needed but I was afraid of running out. (I am looking at changing this to LiFePO4)
• I am using a CotekSD2500-124 2500 W inverter. https://www.cotek.com.tw/product/Pa...ave-Inverter-with-AC-By-pass-Function-SD2500/
• I am using a Xantrex C35 charge controller in Diversion Control Mode. (It sheds unused power to a power resistor in order to prevent over voltage and free-spin on the turbine.
• When the Cabin is not in use, the turbine is shut down and put away. Consequently I added a 20 watt (36Volt Open Circuit) solar pannel to keep the battery charged when not in use.

The is what the turbine/alternator looks like.


Edit: added pic of the turbine.

 

[FilterGuy]

Now to describe my problem: The batteries died in just two years but were in use for only about 10 weeks. I am pretty sure something is wrong with my design for trickle charging when the system is shut down. I am looking at how to solve this for Lead-Acid batteries, but my preference would be to use LiFePO4 batteries and just disconnect them when not in use.

The biggest problem I am facing with LiFePO4 is the low end storage temperature. The cabin is in the mountains and it gets very cold in the winter. Since I have to fly to get to the cabin, taking the batteries home is impractical. You can see some of my discussion about this on the following thread:

LiFePO4 Storage and charging options

Nice Find Will. I really appreciate all the research you do and share!!! I was hoping it would cover low-temp charging but it doesn't. I had always heard "Never charge below 0 deg C (32 Deg F)" and in your videos you talk about keeping the temp a few degrees above freezing for charging...

www.diysolarforum.com

EDIT: Corrected the link to further discussion about cold storage

 

[SCM]

"build a LiFePo4 (with BMS) battery compartment and try to keep the compartment temperature above freezing point by electrical heater or heating trace wire, and below 45C by ventilation in Summer" .
Lead/Acid needs maintenance all the time.

 

[somewhereinusa]

LiFePo can be stored and discharged for long periods of time without losing their charge below freezing. They can't be changed below freezing. It's best if they are stored at 50 to 80% SOC.

 

[SCM]

Yes, that is all correct. And using LiFePo4 at high temperature could reduce the service life. And constant condensation and high RH% is bad to them too.

 

[FilterGuy]

Thanks for the input!
The cabin is in the mountains and the hydro system is shut down during the winter. I hope to be able to store LiFePO4 batteries (totally Disconnected) over the winter and am looking for various ways to do it. During that time the only electric power source is a small solar panel and the battery itself.....and the solar panel is unreliable due to snow and ice covering it.

I dusted of my college thermal-dynamics and calculated that for a small box insulated to R30, I would need about 10 Wh per day to overcome a -20 C temp differential. Because of the thermal mass of the batteries, I can go a few days without the solar panel working, but it gets dicey after that.

Another idea I am toying with is to put (unfrozen) freezer packs in the insulated battery box . I calculated that when 2 liter of freezer packs start to freeze, the phase-change would keep the batteries at the freezing point for several days before the internal temp started to drop again. (but they would also hold the internal temp at the freeze point for several days after the ambient temp went back up) (Note: The freezer packs have a freezing temp a few degrees above the rated minimum storage temp of the batteries)

Other ideas that have come up include heat from bio-mass and storing them in a hole below the frost line. I have not totaly rejected these ideas but I can't figure out a practical way of implementing them.

I would only want to use the battery to heat the box as a last ditch attempt (but would prefer to avoid it all together). Even at only 10Wh/day, the drain could eventually kill the batteries and then I would be storing dead batteries. To do something like this would require active circuits on the batteries to ensure they don't drain to far and that alone will tend to drain them.

The interesting thing is that many of the LiFePO4 cell specs say you can only store LiFePO4 at 0 or -10c, but can discharge at -20c. On the surface these specs seem contradictory. I am trying to get info on what is really going on.

• What is the damage mechanism that is happening in the battery if it is stored below -10C. (Is it lithium formation like happens if you charge at below freezing?)
• Is the ability to discharge at such a low rate due to internal heating or is it some other beneficial chemical reaction of having a current flowing?
• If I discharge a 200aH cell at 0.1 amp for 1000hrs (40 days) while the battery temp is -20c, would that prevent the damage that storing at -20c for 40 days would cause?

If anyone out there has additional ideas or knows what is happening inside the batteries at low temps , please let us know.

 

[SCM]

The LiFePo4 batteries will always have BMS built inside. The BMS will shut down the power completely (without damaging it) when the voltage drops to 10V (for 12V battery), 20V (for 24V battery). It can be stored at lower temperature _down to -10C or even -20C for months, as long as there is no condensation. Because condensation is the enemy of the cells and the BMS inside the LiFePo4.
The BMS inside the battery would prevent it from charging when temperature drops to below freezing point. But the battery would still discharge power until -10C or even -20C. Also higher temperature (over 45C) discharging would shorten the service life, despite some specs allow you to use it until 60C. The temperature switch (or sensor) inside the BMS would prevent it from discharging when the temperature reach 65C.

If you have the solar panel tilted at deeper angle, say over 45 degree, snow accumulation shall not be an issue. if you have the bottom of the panels exposed to the air, the snow would also melt pretty fast in the warm days. In any case, the LiFEPO4 shall survive the winter by itself, as long as you have some kind of temperature control draws the power from LiFePo4 itself.

tips:
you can wake up a dead LiFePo4 after it was self-protected (voltage drops to a few volts) due to low or high voltage protection, or short-circuit current protection. You can connect it to external power source with same nominal voltage to wake it up. The solar panel power will also wake up a sleeping LiFePo4 automatically, i tried and tested it many times. when the sun is shining, a dead LiFePo4 will wake up by itself! but you need a charger like: solar charge maximizing controller from www.enerpak.ca , you could leave the "pre-charge switch" on for the whole winter for this function to work. But Enerpak has 24V system only available currently. The MPPT solar charge controller may also be in the system, but it will also rely on solar charge maximizing controller to wake up in case that the LiFePo4 falls asleep.
The high voltage protection for 24V LiFePo4 is normally 29.2V by BMS inside the battery (prevent you from charging over 29.2V). The solar charge maximizing controller can be set to stop charging the lithium iron phosphate or any other types of battery at 28.8V (adjustable on site). At 28.8V, the 24V LiFePo4 battery would already reach over 95% of its whole capacity.

 

[FilterGuy]

> The LiFePo4 batteries will always have BMS built inside.
This is true for the 'drop-in-replacements' but not necessarily true for DIY systems built from cells..... However, any DIY LiFePO4 battery bank should also have a BMS.

> It can be stored at lower temperature _down to -10C or even -20C for months,
Are you saying this is true for all LiFePO4? The spec sheets on cells give minimum storage temps ranging from 0c to -20c.

Even -20 c (-4 f) can be a problem. At my mountain cabin it is common for the temp to drop to -10f (-23c) and it is not uncommon to see -20f (-28c). The lowest recorded temp was -42 F (-41 c).

> The BMS inside the battery would prevent it from charging when temperature drops to below freezing point.
Not all of the BMS systems have low-temp cut-off (even on the drop-in replacements). Clearly, for my system I will need a low-temp cut-off on charging. That is the easy compared to the low-temp storage problem.

> If you have the solar panel tilted at deeper angle, say over 45 degree, snow accumulation shall not be an issue.
This has been a big unknown for me. I do have the panel at a pretty steep angle so hopefully you are correct. The cabin is remote enough that it is inaccessible in the winter so I have never been there to see how snow and ice affects the panel.

> In any case, the LiFEPO4 shall survive the winter by itself, as long as you have some kind of temperature control draws the power from LiFePo4 itself.
I am not as confident of this as you seem to be. I remain concerned about draining the battery and end up storing a dead battery.

• At the cold temperatures, the power available from the battery drops rapidly.
• Most LiFEPO4 cells store best at less than full charge.... but if I am counting on the power to heat the battery, I would need to start with a full charge.
• The BMS will have a constant drain on the battery.

Having said all of that, If I am not confident I have the worst case covered through other means, I will probably end up doing this. I would set it up to only turn on if it was absolutely necessary.

>you can wake up a dead LiFePo4 after it was self-protected
That is true, but in order to extend it's life, I would avoid ever letting the battery get that low .

 

[Deleted member 783]

Lead/Acid needs maintenance all the time.

Old school lead acid. Sealed lead acid, like GEL batteries, require zero maintenance. Also, not all LiFePo4s have built in BMS. I think it is more the exception than the rule.

 

[Ped]

You only have 20v to charge 24v batteries. Youd be much better off running 2 seperate banks @ 12v. Run one and use other as reserve all through an mppt controller. Because you cant just throw amps into them and be good. You have to reach 14.6v-14.8v and hold that voltage until less than 1 amp is going in to reach 100% or what happens is what you see; they sulfate.

 

[FilterGuy]

You only have 20v to charge 24v batteries. Youd be much better off running 2 seperate banks @ 12v. Run one and use other as reserve all through an mppt controller. Because you cant just throw amps into them and be good. You have to reach 14.6v-14.8v and hold that voltage until less than 1 amp is going in to reach 100% or what happens is what you see; they sulfate.

Sorry, I had my description wrong. (I just corrected it) The turbine outputs a nominal 24 volts. Open circuit, it will drive a much larger voltage, consequently, I have a load shedding charge controller to ensure I don't have an over-voltage situation. (This is the big difference between generators and solar. With generators, you must always have a load of some kind or 1) you get really high voltage and 2) you can destroy the turbine by letting it spin too fast.)

 

[FilterGuy]

Old school lead acid. Sealed lead acid, like GEL batteries, require zero maintenance. Also, not all LiFePo4s have built in BMS. I think it is more the exception than the rule.

That is correct. Sealed lead acid require no physical maintenance. However, they self-discharge quickly so you have to keep them charged. You can't just let them sit for a long period. Furthermore, they are very sensitive to over-charge and and undercharge.

 

[FilterGuy]

When it comes to the cold electrolyte, a fully charged lead acid battery can withstand temperatures down to -33°F (-36°C) before freezing. When fully discharged the electrolyte is basically water so it will freeze at 32°F (0°C).

 

[FilterGuy]

As far as I can tell, up-front cost and low-temp charge & storage characteristics are the only advantages Lead Acid has over LiFePO4.

 

[SCM]

Sorry, I meant flooded lead/acid needs maintenance.
The cell in the lithium iron phosphate battery can be stored near -20C. but it also depends on the BMS in the LiFePo4. Some of components in the BMS may not be kept for too long at -20C, such as capacitors. It should be kept with 50~80% SOC charge if stored for weeks or months. But hopefully, the solar panel shall be able to charge it again before the voltage would become too low.

 

[erik.calco]

relion helped solve this by putting a heater inside the battery that uses charge current to heat until the cells are warm enough to safely charge.

https://relionbattery.com/blog/lithium-battery-cold-weather

You could theoretically replicate what they did by using the solar panel to heat the batteries until a temp is reached, and then only charging after they are warm enough.

What I don't get is why some of these specs out there have a higher storage temp than discharge temp. Need more info on what example happens when a stored LFP is in low temp ranges. Self-discharge gradually decreases as it approaches 0C. Could the problem be a reversal leading to high self discharge? Perhaps contact one of the manufacturers?

One question I have is why even use a SCC to direct a panel to a heating element? Has to be something that can act as a sink for the current that just heats up.

Here is hope:

https://www.grepow.com/page/low-temperature-battery.html

 

[FilterGuy]

My main concern with using the battery to keep itself warm for long term storage is running the battery down... I have not ruled out using the battery power, but I would prefer to use an external energy source.

Since I have the micro-hydro that won't be running in the winter, I only have a small 20W 24 v Panel. My tentative plan is to wire it directly on a resistive heating element with two thermostats (redundancy) in the battery storage box. I will probably use something like this for the heat element.
https://www.amazon.com/24V-Flexible-Polyimide-Heater-Plate/dp/B074SXKPZL. If I decide I want to charge during storage, I would probably use a separate small panel on a small charge controller. This would probably be simpler (More reliable) than trying to switch a panel back and forth.
Also, I would set the solar charge controller max voltage pretty low. (LiFePO4 stores better at lower levels of charge)

I agree that the storage temp and discharge temp specs seem counter intuitive. I have yet to find anything that explains this. I was hoping someone on this forum might have some insight on what is happening at the low temps.

I saw the Grepow LT series as well. Yes, it is promising. I sent them an email asking for price and technical literature but have not heard back.

 

[erik.calco]

I like where you're going, and hope you update us on how the resistive heating elements pan out.

To be clear, Relion isn't using the battery's juice to warm the battery. It is redirecting charge input towards heating until the batteries are ready. So, they would take the charge coming from a SCC to warm the batteries. I really like that concept. No draw on battery at all and not wasting PV or other charge sources at times the battery isn't ready for a charge due to temps. That said, it's a little complicated to do DIY, so completely understand if you skip that option. I just thought it was an interesting change from the solutions we hear of using the battery's reserve to heat itself.

Because they are doing it entirely inside the battery, the SCC is completely unaware. It just thinks it is charging a battery. And that's the part that makes it tricky for DIY.

 

[toothy]

Why not build a root cellar or other earth temp moderated battery room?

 

[FilterGuy]

Why not build a root cellar or other earth temp moderated battery room?

Yes, that would work..... I don't have the equipment to dig a root cellar and don't have the funds to hire someone else to do it. I would love to get a back-hoe in there and do some major digging..... but that is not going to happen.

However, I am considering a type of earth temp moderated system. There is a crawl-space that, for various reasons, would not be a good place for the batteries. However, it is about three feet below ground level. This means the dirt in the center of the crawl space will almost certainly be warmer than the outside air. Without too much work I could run ducting along the ground and maybe even partially bury it. If I throw insulation over the top of the duct and a few feet on either side of the duct, I would have a poor mans 'earth tube'. I could rout that to the insulated lean-to that holds the batteries. The crawl space is vented to the outside, so the air coming through the tube would still get pretty cold, but I am willing to bet it would stay several degrees warmer than the outside air. Consequently it would keep the lean-to a bit warmer than the outside air. This would not be enough by itself but in junction with some of the other ideas, it might be enough to ride out the really cold spells.