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Retevis RT-97 Battery Size Recommendation


Guest Brandon
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Guest Brandon

I am looking to purchase the Retevis RT-97 Repeater. I want to setup the Repeater in a shooting house on my father's property, which is the highest point. I plan to mount the antenna on the tin roof, along with a solar panel (to keep the battery charged), which will power the RT-97. The issue I am running into now is not knowing which size rechargeable battery I should use to operate the RT-97 and safely leave it unattended. Any suggestions or red flags to my ideas would be greatly appreciated.

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It depends on your climate and how much use it will see. 

The RT97 draws about 2 amps on high power and .09 amps when in stand-by.

I am running an RT97 on solar power here in Alaska. During the summer when we have lots of sun a 9 amp hour battery and 30 watt panel do just fine. Everytime I checked on it the battery was at 100% capacity. During the winter we have a few issues that make it more difficult. During winter solstice there is only around 4 hours of sunlight.  We also have cold weather to deal with. Drop a lead acid battery to around 0 degrees F and you are down to around 80% of its capacity. Last winter I had it running on a 30 watt panel and a 14 amp hour battery. It wasn't enough. By November the battery was already being run down to the cut off voltage of 11.2 volts. 75% of last November the repeater was down. We didn't have enough snow to prevent me from getting to the site so I was able to get to it and swap it battery out for a 9 amp hour one I had. Yes it is smaller but I wanted to save the larger, more expensive, battery from damage. It lasted for about a week before going down again. I would come back up after a week or two for a few days and then go down again. From there it only got worse. The solar controller shut down the repeater for the majority of winter. Low voltage and cold temperatures resulted in a frozen battery sometime between December and February. Once frozen the voltage dropped to near zero and the solar controller shut down completely. My repeater site is inaccessible once winter sets in due to snow level and steep grades.

This spring I changed a few things. Keep in mind all my components need to be hiked up to about 2400 feet by hand. The components had to fit in or attach to a backpack or two.

POWER GENERATION:
This spring I added a 50 watt panel. This brought up the solar power to 80 watts total. The 80 watts of panels should generate enough solar energy on a 4 hour cloudy day to replace the 2.2-ish amps that the RT97 uses while in stand-by for 24 hours. I based this on monitoring the output of the panels on an overcast day this summer. I waited until the sun's elevation (as informed by a smartphone app) matched that it in the winter and saw the battery being charged at a rate of about 800 milliamps.  4 hours at 800 is 3.2 amps.  That is 1 extra amp...in theory. 

POWER STORAGE:
I upgraded to a 35 amp hour battery and put it underground by over a foot. Just being a foot underground shields the battery from the extreme highs and lows. At that depth theory says it should be at the avg daily temperature. If this setup works through the winter I am relocating the repeater to an even more remote location and will try to get the battery further down. Being underground also has the benefit of keeping the battery cool in the summer, which in theory, should prolong it's life. This 35 amp hour battery chould, in theory, keep the repeater running in stand-by mode for about 13 days or it could support about 14 hours of non-stop transmission in the winter with ZERO solar input. This factors in a 20% reduction in capacity due to cold temperatures.


With these two upgrades the battery should really never be run low unless we have significant activity on the repeater without any days of decent solar generation. The larger battery helps store "extra" power from the sunny days and/or the additional hours of overcast days before/after winter solstice. This keeps the battery at a higher level of charge. Being kept at high charger levels and buried underground protect it through the colds snaps. 

Since I implemented the changes the repeater has been running 24/7. The past several weeks have been COLD here. The avg daily temperature as been between -5 and 5 degrees fahrenheit. Lows have been down below -20 degrees. For the past two weeks I have connected into the myGMRS national net for about 4 hours on Sundays. This has resulted in about 3 to 4 hours worth of transmission time each Sunday on the repeater as people talk across the nation. So far the battery appears to be doing fine as the repeater has not gone down. Hours of sunlight will continue to decrease through December at which point it will start picking up again. The skies will also start to be clear of clouds more often as we push past mid winter. 

So for me, it looks like 80 watts of solar power and a 35 amp hour battery are needed but again that is due to cold winter conditions with low sun levels. I don't know where you are but if you are in the lower 48 I would say the system could be more like my first attempt, 30 watts solar and a 9 amp hour battery and I would bet a 50 watt solar and 14 amp hour battery would give some extra head room.

I have a few other posts up detailing my experiences with the RT97. If you register you can browse them.

Solar Panel:  
https://www.amazon.com/Renogy-Monocrystalline-Efficiency-Charging-Applications/dp/B07GTH79JP/ref=sr_1_4?crid=2OBPM6JH5RIF2&keywords=50%2Bwatt%2Bsolar%2Bpanel%2Brenogy&qid=1637621917&sprefix=50%2Bwatts%2Bsolar%2Bpanel%2Breno%2Caps%2C318&sr=8-4&th=1

Solar Controller:
https://www.amazon.com/dp/B07Q79TC2L?ref=nb_sb_ss_w_as-ypp-ro-model_ypp_ro_model_k0_1_10&crid=LSOHLRTW8QW2&sprefix=10+amp+sol
 

Battery:
https://www.batteriesplus.com/productdetails/slaa12=35c

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sun.png

Battery-S-12330.pdf

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Guest Brandon
8 hours ago, WRFP399 said:

So for me, it looks like 80 watts of solar power and a 35 amp hour battery are needed but again that is due to cold winter conditions with low sun levels. I don't know where you are but if you are in the lower 48 I would say the system could be more like my first attempt, 30 watts solar and a 9 amp hour battery and I would bet a 50 watt solar and 14 amp hour battery would give some extra head room.

I have a few other posts up detailing my experiences with the RT97. If you register you can browse them.

WRFP399, I appreciate your advice and your experience with this matter. This site will be setup in central Alabama, so I definitely won’t be getting the temperatures experienced in Alaska. I also appreciate you placing links to the solar panel and controller into your post. I haven’t registered for a license yet, because this whole project was meant to be utilized by my father; however, if I’m going to be the one servicing and setting all this up, I probably just need to buy the license so I can get involved with this forum. 
 

I did notice on one of the other threads that you commented on about someone setting up a construction site in the Blue Ridge valley. I pulled a lot of great information from that feed.

 

My biggest concern was the battery size needed to power the RT-97 and the Solar Panel size to keep the battery maintenance free. I believe, just to be on the safe side, that I will run him a 50w panel on a 14amp hour battery.

 

One other question I have is the type of battery recommended (i.e. Gel, Lithium, Sel, Lead, etc.)? I’ve been looking on Amazon, and those are a few options I have noticed. Any suggestions would be most helpful. Thanks again for your wisdom, WRFP399.

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11 hours ago, Guest Brandon said:

WRFP399, I appreciate your advice and your experience with this matter. This site will be setup in central Alabama, so I definitely won’t be getting the temperatures experienced in Alaska. I also appreciate you placing links to the solar panel and controller into your post. I haven’t registered for a license yet, because this whole project was meant to be utilized by my father; however, if I’m going to be the one servicing and setting all this up, I probably just need to buy the license so I can get involved with this forum. 
 

I did notice on one of the other threads that you commented on about someone setting up a construction site in the Blue Ridge valley. I pulled a lot of great information from that feed.

 

My biggest concern was the battery size needed to power the RT-97 and the Solar Panel size to keep the battery maintenance free. I believe, just to be on the safe side, that I will run him a 50w panel on a 14amp hour battery.

 

One other question I have is the type of battery recommended (i.e. Gel, Lithium, Sel, Lead, etc.)? I’ve been looking on Amazon, and those are a few options I have noticed. Any suggestions would be most helpful. Thanks again for your wisdom, WRFP399.

Do everything you can to keep the panel in full sun. I chose a sealed lead acid battery. It is an AGM. I went with lead acid because they are forgiving with cold temperature charging. I went with sealed so I could transport it without the worry if leaks.

 

You also need to match the charger to the battery. You can't charge a lithium battery with a controller for a lead acid.

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Guest Brandon
On 11/23/2021 at 1:03 PM, WRFP399 said:

Do everything you can to keep the panel in full sun. I chose a sealed lead acid battery. It is an AGM. I went with lead acid because they are forgiving with cold temperature charging. I went with sealed so I could transport it without the worry if leaks.

 

You also need to match the charger to the battery. You can't charge a lithium battery with a controller for a lead acid.

Thanks for the advice. I look forward to getting this repeater station setup and running.

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  • 4 weeks later...

As an update we have been heavily overcast here for about 2 weeks. So much so it appears the panels are making little power.  I have connected into myGMRS nets on Sundays, a Wed Tech Net and the Sat Family Fun Net with Santa. Each was about 2 to 4 hours of TX time each. On Sunday, 12-19-21, at the start of the net the repeater went off-line. I assume the battery protection circuit kicked in. This is the first time the repeater went down this year.

Yesterday, 12-20-21, it was heavily snowing all day, so little to no charge. Today, 12-21-21 (Winter Solstice) we have direct sun. The sun isn't reaching the panels until around 11:00AM and it will be gone by 4:00PM. The sun is also at a very low angle. This week it seems like I have a chance for 2 mostly sunny days and a few partly cloudy ones. Guess we will see if that is enough to bring the battery back up to 12.6 volts from 11.2 and turn the repeater back on.

On the plus side we are going to be gaining daylight at a rate of about 4 minutes a day from here on. By February we will be back to around 8 hours of sunlight. 

I attached a photo I took around 11:15AM today of the repeater site. The ridge line you see runs east/west so the face of the ridge is nearly due south. The shadow you see is caused by the opposing mountain blocking the sun. The sun is able to get the site around 11:00AM due to a valley that runs north/south on the east side of the mountain. As the sun continues to rise it is able to stay over the top of the mountain. It kind of follows the contour the mountain if you will. I attached a time-lapse image I stole illustrating the suns arc during winter for reference.

For those asking why I didn't put it up higher, it is b/c it's too hard to get to (the grade is steeper than it looks) and the summit has no good place for it. No poles to place it on to keep the panels above the snow line and it would be too easy for hikers to tamper with it.  This spring I am going to relocate it further west down the ridge line several miles. This will increase it's RF footprint into populated area and pull it further from hikers.

Anyway, this has been a fun project for me.

263816424_5028734370499749_6963745088218800486_n.jpg

DSC_0627.jpg

12-21-2021 Weather.png

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One thing with solar panel is even ONE cell in the series string is blocked, bird poop - tree leaves etc. for example, it kills the output from the whole string. All the panels I’ve ever seen are single series string of cells.

Also at low temperatures the battery capacity is noticeably reduced. You should take that into account when sizing the battery.

Also you’ll likely get about half the rated capacity before the battery voltage drops too low for reliable operation too.

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I have the two panels running in parallel...hopefully that helps some with partial shading when it happens. So far the manufacturer says at -10 degrees C I should be around 80% capacity at a .05C discharge. Look a few posts above for more info on that.

 

We shall see if it powers back up this week.

 

If I wasn't in Alaska and was further south I am confident this system would be running flawlessly with just the 50 watt panel.

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  • 3 weeks later...

Update: Has not come back online. I don't know why. I don't know if the battery froze, the solor controller messed up, or the RT itself is down. I doubt the battery is frozen based on the following experiment. I also doubt the panels are not producing enough power to charge up the 35 amp hour battery.

 

 We have had a week straight of clear sky's and it's getting at least 3 hours of good sun a day. I conducted an experiment. Iook a 14 amp hour battery, drained it with a 2 amp load. It drained until the the cut off circuit of the same brand/model solar controller cut off the load (11.2 volts when cut off and rose to 11.8 without a load). I then put the battery outside in -5 to -20 degree F weather for 48 hours. The battery did not freeze.

 

Today I put a 30 watt solar panel on it to see how it reacts. 

 

The 30 watt panel quickly, as in a minute or so, reached a voltage of 14.5 and the solar controller turned the load back on as it saw a voltage greater than 12.6. The battery was seeing a .75 amp current at 14.25 volts flowing to it from the solar controller.

 

So the repeater should have come back online, at least during sunlight hours. I may be able to snow shoe to it to see what is going on but I don't know. May have to wait for spring that (April/May)

 

 

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  • 4 weeks later...

Tried to get to it to see what was going. That ain't gonna happen for a few more months.

After research and spending time on solar forums I have come up with a Beta Test 3.0 setup for this year.

It is basically all the same but instead of the cheap PWM controller I got an Epever Tracer2606BP MPPT. This controller allows me to have more control over the set points. This allows me to keep the low voltage cut off higher. It also has temperature compensation to offset for the cold issues. I can further set the load, the RT97, to turn off when it isn't being used between midnight and 6 AM.

 

So I set up some parameters for daily consumption as follows:

10 min of transmission time a day, 17 hours and 50 min of standby and 6 hours off. This uses 29.64 watts/hours of power. The transmission times is very conservative to reflect the solar power issues in the dead of winter here. Outside of December transmission time can dramatically increase and becomes unlimited during the summer months.

 

To break even with that level of consumption in December I need to have enough panels for the low light. Solar insolation figures are .54 in December. Running some conservative efficiency figure of .72 to take into account losses of the pa el, charge controller and battery I need 76 watts of panels in December to break even.

Running a 35 amp hour battery gives me some headroom. Remove 20% capacity for cold weather. Shutting off the load at 11.8 volts means I consumed about 70% of the usable capacity. That is about 253 watt hours in the battery giving me 8.5 days of use. 

Once the load shuts down I need to have enough power in the battery to keep the charge controller going until it gets sun. With about 100 watt hours remaining in the battery that gives me about 24 days until it runs the battery to 10.5 volts. That should be plenty of time for a day or two of sun.

Based on my real work measurements, each sunny day at winter solstice makes about 135 watt hours, so about 3 sunny days are needed to push it back to "full charge" from full depletion.

Once we are away from December I have more than enough power. It's just that month of December.

___________________________________

Rates of Power Consumption:
Transmission = .17 hours x 2.15 amps x 12.9 volts = 4.72 watt hours
Standby = 17.83 hours x .104 amps x 12.9 volts = 23.92 watt hours
Off = 6 hours x .013 amps x 12.9 volts = 1.00 watt hours

Total Daily Watt Hours = 29.64
___________________________________

Solar Panel/Charge Controller/Battery Efficiency = .72
___________________________________

Avg Solar Insolence Per Day
November: 1.34
December: 0.54
January: 0.91
___________________________________

 Minimum Panel Sizes To Cover Power Consumptions:  
November: 30.82 watts
December: 76.18 watts
January: 45.35 watts
June: 9.71 watts
___________________________________

Formula (Totally taken from the solar forums): https://forum.solar-electric.com/discussion/357176/help-with-low-power-mountain-top-solar-radio-repeater#latest

Watthours Consumed x (1/DC System Efficiency) x (1/Solar Insolence) = Panel Size Needed

 

 

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I would be a bit more conservative on the solar insolation value used in your calculations. The charts I've seen from NREL, National Renewable Energy Labs, had a margin of error of about 10 percent. Also your eye may not notice it but small about of dust and haze in the air can result in a noticeable reduction in panel output.

https://www.nrel.gov/gis/solar.html

On the battery side the general recommendation is NOT to discharge a Lead Acid battery below 50 percent of capacity. Doing so can result is a large reduction in the number of charge/discharge cycles the battery can support before it become unusable.

You also DON'T want to let a Lead Acid battery sit around for very long at anything less than full charge. In a partially charged state the Lead Sulfide is porous but then becomes less so over time. When the acid can no longer penetrate the Sulfide it can't be converted back to Lead and Lead Peroxide during the charge cycle resulting in a permanent reduction in battery capacity.  

Most people don't consider what happens when a Lead Acid Battery is charged too fast. Some of the water in the electrolyte is converted into hydrogen and oxygen gases. Even in a so-called "sealed" battery there are vents to release gas build up. Once that gas is released it can't be converted back to liquid water, yes there is a complicated chemical process in GEL and AGM batteries that can do this, but only if the gases generated is done slowly.  If you look at a some of the smaller battery specification sheets this max charge rate is specified. Of course with liquid filled batteries this isn't a problem since you can always add more distilled water.

https://datasheet.octopart.com/NP7-12FR-EnerSys-datasheet-32762402.pdf

For the above example a 7 amp-hour battery the max recommended charge current is 1.75 amps. At a terminal voltage of 14.4 VDC the max power you can safely use for charging is 1.75 X 14.4 =  25.2 watts. I just cringe when I see guys selling so-called solar power generators at swaps with these small batteries and have a 50 to 100 watt PV panel connected to them.

https://www.power-sonic.com/wp-content/uploads/2018/12/Technical-Manual.pdf

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