Hi John, Why do you say that unstable input power would affect longevity of the cells? What do you think happens under driving conditions? As long as you carefully monitor (with that BMS) that none of the individual cell voltages do not go over the max charge voltage, you can throw what you want towards the cells and they will happily absorb it. Heck, you have a 280Ah pack and only 52A of charging current. As long as the cells are not stone cold (then you must charge them slow) they will not have a problem charging at less than 0.2 C. The solar array is quite good as charger actually, because it behaves as current source, following the rising pack voltage. For the most time your pack will be at 154V (3.2V per cell) so the 172 will be pulled down 18V when connected to the pack, this is only 10% so not a big loss.
I know there has been quite some discussions about max charge voltage of LiFePO4, some say stay below 3.45V while others promote goin up to 3.7V. Read the specs of your cells carefully and make sure your BMS can stop the charging automatically when one cell reaches max. Then, there may follow a period of balance charging where you insert a resistor in series with the pack such that the charge current is less than the balancing current of the BMS. For example if your array at ambient temperature has an open voltage of 180V and your pack is at 160V while you want to pull all cells to 3.45V balanced for a total pack voltage of 165V, and your BMS can balance cells at 1/2 Amp for example, the you can insert a 40 Ohms 10Watt resistor to bridge the 20V difference between pack and solar array while allowing not more than 1/2 Amp to flow, so the BMS can absorb (bypass) that current in the highest cell(s) so they do not go higher and continue to charge the lower cells at that current until they too reach max voltage cut off. Hope this helps, Cor. On Fri, Jan 12, 2024, 7:21 PM John McIntire via EV <ev@lists.evdl.org> wrote: > Having read this list for several years now, and commented occasionally, I > know there some electrical engineers out there. It is to you I address this > problem. > > I am now the proud owner of a '96 Solectria Force which will be receiving > a new battery made up of 48, 280A LiFePo4 prismatic cells. The Force was > designed to run at 156v (13 lead acid batteries). My primary power source > is a 16 panel, 8.5 Kw PV array. Max voltage under load is 43v/panel at 13A. > There is no back up power as I am not grid tied. I do have a small domestic > 24v Li battery to run my shop. The Force battery will be equipped with a > robust BMS. > > I can configure the panels to provide 172v at 52A in full sun but I know > that the voltage and amp will be unstable which does not promote longevity > in Li cells. Normal circumstances have a charge controller between the > battery and the array to even out the voltage. Normal charge controllers > are 48v or less. I would like to find or build a buck/boost circuit that > would match the battery. > > I think I could break the Force battery into 3, 16s packs, charge them > parallel at 48v+ and then reconnect in series for discharge. I think 2 > contactors per pack along with appropriate safety devices would do the job > but that seems cumbersome at best if workable. > > I am not an engineer. I do have 30 years experience with automotive > electronics. Since partial retirement, over the past 15 years I have > converted an S-10 and 2 tractors to run as EVs. So I am somewhat familiar > with the concepts and pitfalls. > > I would appreciate your thoughts and suggestions. Thanks in advance. > > John M > > "Money doesn't talk, it swears"--Bob Dylan > _______________________________________________ > Address messages to ev@lists.evdl.org > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20240112/54ed528d/attachment.htm> _______________________________________________ Address messages to ev@lists.evdl.org No other addresses in TO and CC fields HELP: http://www.evdl.org/help/
