LiPo charging from PV - Page 1
Forum section: electronics / beginners

author: Cside
author_rank: Regular Contributor
timestamp: August 18, 2014, 07:37:04 am
content: My latest contraption has the following proposed criteria1. Use lipo batteres as supplied (3.7v, multiples of in Series or parallel is OK)2. Charge from thin film solar with Vtypical at 7.2vDue to weight constraint we prefer not to use lead acid.The technical questions are as follows:1. is it OK to keep lipo batteries topped up via solar intermittantly - when light is available - or must they adhere to a stricter charging regime of typical Constant current - > constant voltage ?2. What is the most efficient way to limit current from the PV cell (this maximizes efficiency thanks to non-linear power curves ) ?3. Safety - as always - a concern.any advice of experiences appreciated.

author: Simon
author_rank: Unknown
timestamp: August 18, 2014, 07:41:47 am
content: I think the charge curve for a lipo is something like a constant current for a certain time and then a constant voltage.For your current limiting from the solar panel with best efficiency you want to use a switchmode power supply that outputs a constant current although you will also need to be able to control the voltage so you will have two feedback controls.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2014, 09:17:15 am
content: Quote from: Cside on August 18, 2014, 07:37:04 amBoth. The peak voltage and peak current need to be controlled but the batteries may be charged at any time. If multiple cells are used in series, then charge balancing should be used.QuoteMany regulators support output current limiting beyond current fault protection which is enough for a battery charger but power point tracking requires a switching regulator and separate control loop.QuoteI might include an over voltage shutdown separate from the voltage control loop because over charging can be catastrophic. If cells are charged in series, charge balancing should be used. Thermal protection of the charger and battery might be appropriate.

author: Cside
author_rank: Regular Contributor
timestamp: August 18, 2014, 09:34:03 am
content: Quote from: David Hess on August 18, 2014, 09:17:15 amFantastic response thans. "power point tracking" was the term I was looking for. What exactly is charge balancing? Does this mean the actual charging alternates between cells? or that a separate system (ie, a dedicated charging chip) for each cell?My concern now has become much about safety. Probably li-po isn't the way to go, I may go Nicad. But It would be nice to support lipo anyway. And as awlays, always good to build something unfamiliar to you.

author: coppice
author_rank: Super Contributor
timestamp: August 18, 2014, 09:55:32 am
content: Quote from: Cside on August 18, 2014, 09:34:03 amDo they still allow NiCads to be sold where you live, or did you really mean NiH?LiPo batteries certainly have issues, demanding great care, but LiFePO4 batteries are much more benign, and readily available. They aren't as light as LiPo batteries, but they are lighter than most battery types.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2014, 03:07:04 pm
content: Quote from: Cside on August 18, 2014, 09:34:03 amPower point tracking allows maximum power to be delivered from the source while the charger is constant current limited on the output. I have seen simple implementations which do this in the past which do not even require any digital logic but do not have any links handy.QuoteWhen connected in series, individual battery cells do not maintain identical charge because of variations in construction and environment so over many charge and discharge cycles, a point may be reached where some cells are being over or under charged which is particularly bad for lithium chemistries. Charge balancing redistributes charge between the cells so that they are all charged evenly. With lithium cells, this amounts to maintaining the same peak voltage on all cells at the end of the charging cycle. Supercapacitors also require charge balancing for long life.QuoteI think lithium iron phosphate cells are tougher and safer than the other lithium chemistries. NiCd and NiMH are more complex to charge because they require some type of end of charge detection although charge balancing is simpler for them.

author: Simon
author_rank: Unknown
timestamp: August 18, 2014, 03:43:34 pm
content: MPPT for solar panels generally involves loading the panel at the right power to get the most watts by controlling the load. In practical terms for charging lead acid batteries this means that you use a SMPS topology that ensures the most current is going through. A solar panel will output more current at lower voltage, but a lead acid battery needs more voltage to charge more current (within safe limits) so if you were charging a lead acid battery you would try to get as much current through as possible by contantly increasing the voltage or decreasing it and checking what happens in order to keep track of battery charge condition and environment conditions on the panel.

author: Cside
author_rank: Regular Contributor
timestamp: August 19, 2014, 01:09:06 am
content: Quote from: David Hess on August 18, 2014, 03:07:04 pmWait hangon, a good PV cell will have its Pmax somewhere near 85% of its short circuit current and open circuit voltage. But if the cell is being intermittantly shadowed by clouds or whatever then surely it wont be able to maintain its peak power throughout the full charge so the current will drop intermittantly. This is my concern with using PV directly to charge LiPo which expliciply requires constant current for half its charging cycle.As for multi cell balancing, Are you saying that the constant voltage step is maintained for longer than normal (1 cell) and that this balances the cell? It sounds plausible.

author: Mr.B
author_rank: Unknown
timestamp: August 19, 2014, 01:43:40 am
content: If you wanted to use only one LiPo battery (3.7v) and a PV that delivers less than 6.5v, then I would use a dedicated LiPo battery charger chip.I have built numerous circuits using both the CN3065 and the CN3083.Datasheet for CN3065 attached.The CN3065 is available from SeeedStudio.In a typical application I have used this chip, PV, 3.3v LDO, Atmega328P-AU, NRF24L01+ Radio - bingo, remote solar powered environmental sensor unit.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2014, 01:53:58 am
content: Quote from: Cside on August 19, 2014, 01:09:06 amConstant current just means the current during the initial part of the charge cycle has to be limited. It can be lower than the maximum charging current for the battery (and usually is) and it may vary over time.QuoteCharge balancing for lithium batteries amounts to limiting the maximum voltage for individual cells in the battery during charging. Usually this is done by shunting current around each cell as it reaches its maximum voltage but if you have just two cells in series, then an easy way to do this is to have the charger drive the junction between the two cells to one half of the total voltage across the battery.

author: Cside
author_rank: Regular Contributor
timestamp: August 19, 2014, 02:06:20 am
content: Quote from: David Hess on August 19, 2014, 01:53:58 amAhh that explains it. I was worried that it needed to maintain a real constant current.Are there dedicated chargers for this kind of multi-cell charging?Now how about parallel charging? Im thinking of buying a pack of say 4-10 lipo batteries and building a parallel connected pack. THis way all the cells are the same age/capacity etc. If I do this then would it be OK to charge it in the usual way as an ordinary lipo cell?

author: Mr.B
author_rank: Unknown
timestamp: August 19, 2014, 02:18:43 am
content: All the documentation i have read about LiPo recommends against connecting directly in parallel.The batteries are usually factory matched when paralleled in the same pack.I have seen 4 bank charger PCBs on fleabay before,but I am sure they were not compatible with a PV.What is your application?Why do you need lots of batteries in parallel.You can get 6000mAh LiPo batteries at a good price.You are going to need a pretty big PV to charge something that big even in a full day of sunshine.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2014, 04:03:55 am
content: Quote from: Cside on August 19, 2014, 02:06:20 amThere are but I did a quick search and did not find any that seemed appropriate.QuoteIt is better but not ideal.

author: Cside
author_rank: Regular Contributor
timestamp: August 19, 2014, 05:13:39 am
content: Thanks to all who replied.I think I have heard enough and will avoid doing anything dodgy.