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Forum Index : Solar : Modding charge controllers for LiFePO4
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yahoo2 Guru  Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi All, I am looking for opinions, theories and ideas on what will work well for solar charging LiFePO4 battery banks using older more basic charge controllers. Looking mainly at 12-48 volt range for PWM and MPPT charge controllers, perhaps leave the 70+ volt DC and AC coupled systems for another topic. Also if you are aware of something that could possibly work well straight off the shelf feel free to post the brand and model number so we can check it out. I will do a post of what basic criteria I think a charge controller needs to meet for LiFePO4 charging and perhaps a couple of examples of some of the stumbling blocks where existing lead acid controllers are falling down. I plan to try a simple mod to a fairly common controller in the next month, so fingers crossed that it wont be a complete disaster and sometime in the next six months I will post some setting adjustments for a more advanced high end MPPT unit. cheers yahoo I'm confused, no wait... maybe I'm not... |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Hi Yahoo, here's what I have been using for nearly two years and which works well enough for me: GSL model MPPT60-1 This unit was purchased when I just had a vented LA battery bank. The LA batteries are still connected, as a backup when the LYP (note different lithium technology) battery bank should trip out due the low voltage relay sensing. This has not happen yet. So my solar power now comes from the Winston 200Ah LYP batteries and the MPPT unit equalises them to 28.9V (24V system). I know that is not optimal as the batteries do not get 100% charged but it works for me as it is. Since they never get deep discharged (<50%) either I believe their cycle life is quite long, perhaps 10 years (I hope). The advantage with this MPPT controller is that it suits the LA battery bank in parallel as well and just keeps it fully charged. With 2KW of solar panels I have seen 59 Amps charging The LYP bank which does not seem to worry the GSL unit despite being so close to its maximum rating. Klaus |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I think the important factor here is some form of logging the battery data to see what happens over time, with what sort of charging system. Tinker has a logging system in place and is able to see what works and how well. Without logging battery data its really a mugs game to what works, as we are never present to see the high and low points on the average day, so we think all is well when it may well not be. A quick view of a daily graph on a computer can tell all, good or bad. Pete. Sometimes it just works |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Thanks Klaus, that's great! You may be closer to 100% charge than you realize, the little test I done the other day where I bypassed the controller @ 3.45volts/cell (27.6 on a 24v system) it flew past 30 volts in 40 seconds I would guesstimate that to be a 10th of an Amp. Of course, your LYP's could be different but I think the combination of reducing current and the low resistance of the batteries means we are getting them a lot closer to full with solar than the EV guys are with high current mains powered chargers at the same voltages. Assuming the cells are balanced it should be theoretically possible to be 100% charged at 3.5 volts/cell. I'm confused, no wait... maybe I'm not... |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
LIFePO4 CHARGING CRITERIA I'm finding it a bit hard to find the words to simply explain charging LiFePO4 battery banks. the best I can think of is to use the example of a LiFePO4 mains charger. They charge to 3.65 volts per cell.........! that is pretty much it! 3.5volts/cell is regarded as fully charged 3.6volts is the point where unbalanced cells start to drift up or down in voltage 3.7volts is regarded as a good safe upper voltage limit. So in an ideal world I would set a solar controller to 3.65volts/cell. No equalization cycle! No temperature Compensation! very low end current or no end current with a time limit set. There a few that have the custom settings to do this, but not many. The risk of aiming for 3.65volts/cell with a temperature compensating controller is that on cold days it would be quite possible for the voltage to climb above 3.7volts. The other challenge is that on a lot of controllers the only non equalizing option is the Gel battery setting and the end voltage is around the 3.55volts/cell, if this is factory preset and not user adjustable then with temp compensation it will get as low as 3.4volts/cell on a hot day. That's it in a nutshell. I have included a copy of my cheat sheet as a quick reference from cell volts to bank volts.  I'm confused, no wait... maybe I'm not... |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Yahoo Based on your figures the old BP solarreg is spot but it has manual equalisation switch that takes it to 32 volts, the other surge 24volt unit has 30.5 equalization auto cycle so should with in full auto, the other 80 amp ones have programmable equalization cycles, so should work as well. From what I have see nit is better to of go for the final few amps and cause balance problems. All the behest Bob Foolin Around |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
yahoo2, Tracer do a series of 10, 20, 30 and 40A MPPT chargers. I have the 20A variant set to GEL, as my tired lead-acid batteries are really chewing through the water. However, your comment about temperature compensation resulted in an "idea". These units have a thermal sensor on the front panel. I was thinking of removing it from the unit and put it in with the lead- acid batteries, but then when you said "no thermal compensation is required for LiFePO4", I thought just maybe one could replace this resistive thermal sensor with a pot and then you would be able to shift the various voltages around, ie float down from 27.6 to the 27.2V you state above. Could be a lot easier than reverse-engineering the rest of the circuitry. Here are the specs for these units: Tracer MPPT specs Cheers, Dave |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
cheers Dave, the only reservation about fiddling with the case temp sensor is that I am worried about how that will affect the derating of the mosfets. I have a gut feeling that the mosfet junction temperature is measured through a diode and the ambient (case) temperature sensor is used to calculate heat dissipation, however my severe lack of knowledge in this area means I am flying blind. The controller I will be testing has solder pads for a remote temp sensor and battery voltage sense wires. I hope to use both to alter it's sense of reality just a little bit. The tricky bit is going to be increasing the absorption volts as high as I can while keeping the float volts as close to standard as possible. Using a pot to clamp the remote temp reading will be crucial in making a controller LFP friendly. I'm confused, no wait... maybe I'm not... |
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norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
The Outback MX60 will allow you to alter, Bulk, Absorption, float and equalise voltages, but because it`s made for lead-acid not capable of balancing each cell. Will be interesting to see how the life of the batteries go without balancing. From experience with the small batteries used in RC, balancing is recommended as a must. We come from the land downunder. Vic |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I guess I haven't made this quite clear enough. It can be a little tricky if you are not standing there looking at a bank in operation. At the present time almost all LFP off-grid battery banks have cell balancing and safety shutdown relay(s) that function independently of the charging operation. The only function that the solar controller has to perform is to get the voltage of the bank past the 100% full mark and into the zone that the cell voltages start to spread (between 3.6 and 3.7 volts per cell) so that the balancing hardware has a chance to do its job. while I was researching this stuff I grumbled a lot about not being able to buy an all in one box, I have changed my mind since then. I really like the Battery management systems (BMS) and I think we have missed a trick in the past by not having something like this as part of our system kit. I'm confused, no wait... maybe I'm not... |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
yahoo2, My initial reaction is that the thermal sensor is only used to modify the -3mV/C temp compensation. I was puzzled by the fact it is mounted on the front panel. External air does seem to impact more than any internal heating. My unit runs cool at 10A. Maybe, higher power units might use the internal temperature to modify the duty cycle or ? to protect the power FETs. Do you mean for lead-acid or LiFePO4 systems? Dave |
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