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Forum Index : Microcontroller and PC projects : Solar Charging Li-Ion batteries
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OA47 Guru  Joined: 11/04/2012 Location: AustraliaPosts: 986 |
As mentioned in earlier posts I have some MMs radio connected in a star network for in-field data collection. I have been using 12 Volt SLA batteries that are charged by a 10W solar panel. I have been experiencing battery failure which I believe is caused by evaporation of the fluid in the SLA batteries due to consistent dry and hot temperatures. I have been considering using Li-Ion batteries (3*3000mAh) to replace the SLA batteries but I am worried if my simple charging cct can be utilised without too much butchery to the PCB's. Here is the circuit I am currently using:  Firstly R4 and R2 set the voltage presented to the battery (allowing for D2). Charging current is limited by the transistor turning on and Imax can be adjusted with the value of R1. Any discussion appreciated. OA47 |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
G'Day. Back when I was a lad I was involved with collecting data on a South Australian farm. That meant below zero in winter, condensation, extreme heat in summer, insects, rodents ,etc but I did have mains power most of the time. It seemed to me that a car fridge was cheap and would provide protection from all of the above. I think I was using the 16F84 at the time and it all seemed reasonable but I never got to try it. The thought of charging these modern batteries scares the Hell out of me. Peter |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 986 |
PeterB. The major issue with your thoughts is that I have to make the units as compact as possible as they share the rows in the field with machinery as well as the list of hazards that you mention. OA47 |
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zaphod Regular Member  Joined: 03/06/2018 Location: United KingdomPosts: 93 |
I don't know the spec of your batteries but it's normal to have temperature compensation of the float voltage especially as it sounds like your operating temperature range is wide, this might be contributing to your problem ? Cheers Roger 1Kwp DIY PV + Woodburner + Rainwater scavanger :) |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5091 |
Dear OA47, Please be aware that a 3 cell lithium ion battery will require a balancer. Even if the cells are matched to 1% of their capacity, your solar charger will charge them every day (*). Within a year the unbalance will destroy cells. A balancer is not a big complex circuit as long as the capacitance of the batteries is low, but or 3000mA (do you actually need that much to survive the night ?). Here is a picture of a 3S balancer I use for my model airplanes.  From looking at your circuit diagram your charger will charge with 750mA max (that is perfectly fine for 3000mA LiPo). The voltage should change from 13.8V to 12.6V for 3 cell LiPo (3 x 4.2V). Change R4 from 1.2K to 1.1K (1.08k would be optimal, but 1.1K allows for 0.2V drop across D2). It is interesting to find out why the SLA batteries died. They are quite robust, far more robust than LiPo batteries. They can die of overcharging (not expected with 13.8V) or deep discharge. Well...both are also killing LiPo's. In fact, LiPo's will degrade fast if discharged below 3V per cell (9V for 3 cell). You will need to make provisions to prevent that from happening (cut off power). Regards, Volhout The critical parameter is charging voltage. A LiPo cell should never be charged above 4.2V. If there is a difference in capacitance between cells (in series), the difference in cell voltage will get bigger with every charge-discharge cycle. That is why you may not need a balancer in some applications. If a LiPo is only designed to get charged 10x in it's lifetime (i.e. once per year) you may not need a balancer. But at 365 charges per year....you do. PicomiteVGA PETSCII ROBOTS |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 986 |
The units have an average current draw of 6 mA as the LoRa radios are in listening mode and the 12V probe is always attached. I have been using 1.2Ah SLA but I have found that long periods (weeks) of overcast weather and leaf obstruction of the solar panels have drained the batteries. OA47 |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
Volhout. I would have thought that all batteries i.e. stack of cells, would have your balancing circuit built in however there could be problems if you then series batteries to make higher voltages. OA47. It is possible to buy these batteries as direct replacements for the popular SLA types. quote: "You can even use the existing charger". How about doing what the opal miners do ? Peter |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5091 |
@PeterB: 12V laptop batteries have the balancer build in, in most cases implemented inside the chip that also measures the charge, and controls the shut-off FET (in case the battery voltage is drained below 9V this chip disconnects the batteries from the laptop). The cheaper LiPo batteries do not have a build in balancer. They use chargers with build in balancers. These batteries typically have a large connector with 2 pins, capable of supplying the high currents, and a second connector with 4,5,6,7 etc. pins that provide connection to the individual cells for balancing.  Please be informed that laptop batteries (with controller build in) require software to be usable. Without software the only thing they are usefull for is paperweight. Regards, Volhout PicomiteVGA PETSCII ROBOTS |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
Volhout: If you Google something like "Li ion battery to replace SLA" you will see what I meant. The technology is said to be LiFePO4 whatever that is. My chemistry was never good however there does seem to be quite a lot of work going on in this area. |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 986 |
Wondering if I should put some sort of NTC across R4 to lower the charging of the SLA at higher temperatures? OA47 |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 655 |
OA47: The voltage of a SLA should increase with temperature I think. Can I assume you don't like my idea of going underground, are you worried about rabbits ? Peter EDIT Nope I got that wrong. -3mv/degree. Sorry. Peter |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 986 |
PeterB, all good ideas but I have to be very careful when I install the 40cm probe as to not disturb the surrounding area as this will affect the moisture, salinity and temperature readings that are taken at 10 cm depths. If I was to bury the unit some distance away from the probe it would end up being surrounded by water and various fertilising chemicals as the plants are drip fed. I would still need the solar panel and antenna above the plant height. I did google some replacement LIPO batteries and went looking at the specifications. I was greeted with: Must use LiFePO4 charging circuit. Do not use Lead Acid charger to charge this battery. [IMPORTANT: Batteries should be charged with a compatible LiFePO4 charger. It is very important to use a LiFePO4 charger for charging LiFePO4 batteries, and not a lead acid charger.] OA47 |
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