If by "let them go" you mean keep that voltage on the pack even after fully charged, then your voltage is not a good ideal. 12 volt LiFePo pack should be floated at 13.35 volts. There is very little energy above float anyways, so if you are not going to use a multi-stage charger, then 13.5 would be the highest I would float charge a 12 volt lifepo pack.
Balance you pack well before installing it, and then you can probable get away with just voltage monitoring at the cell level. Though my BMS does do active balancing, it never has to with the charge and float voltages set correctly.
I personally avoid paralleling batteries of any type lithium or lead. The fastest way to go out of balance on any pack is to have strings in parallel. It is a lot harder for a pack in series to drift. If you go the parallel route make sure you use larger gauge cable on the parallel joins and use separate chargers for each series string.
I have had my 48 volt LiFePo pack in use for 7 years. It still has 100% capacity and has been absolutely trouble free. The only maintenance I do is: I take a multimeter into the engine room once every 6 months or so and check the cell voltages, and once a year I put the pack under heavy load and use a thermal imager to make sure all the interconnects have similar heat signatures. If you don't have a thermal imager, just check the lug tightness on each interconnect instead.
To be absolutely honest I love my lithium pack, it has lasted longer and been more trouble free then any Lead pack I have ever owned. I would never go back to lead.
As to your plan, 100 ah at 12 volts is only 1200 watts total. That is not going to last very long behind a 3000 watt inverter. But if you are doing this just to get experience, have fun. LiFePo is a great option for a house bank. It does not care if you leave it half charged for days and as long as you have your inverter cut off set at 11 volts, and use the charge voltages about, you should get a long and happy life out of the pack.
If you do ever get solar on the boat, lithium packs take a charge to full with much less wasted energy.
On Sep 13, 2018, at 3:22 PM, oak oak_box@yahoo.com [electricboats] <electricboats@yahoogroups.com> wrote:
I currently have a gas powered cabin cruiser, with a gas powered generator - that may be dead at this point.Rather than replacing the generator, or spending a LOT of money on questionable mechanics over the next couple of years... I'm thinking about replacing the generator with a bank of LiFePO4 batteries and a good sized inverter.My current expectation is that while I'm out away from shorepower, I won't be gone that long, and I won't need a tremendous amount of 120V AC power. Eventually, my long term goal would be to build up the battery bank to the point where I could run the boat's AC for up to 4-6 hours while I'm out on the water... I should be able to get 3000W inverter without spending a fortune, and that should cover anything I'd want to run on the boat (once I build up the battery bank...).This project is just to get things started and get more familiar with lithium batteries, etc...My thought was to start with four CALB 100Ah cells to generate the 12V for the inverter.BMS: I'm comfortable playing with arduinos.* I was thinking of using an Arduino Nano (8 analog input ports) to monitor the battery. (voltage dividers to get the battery voltages at each point of the string within the range of the Arduino)* I can set up the "BMS" to monitor the voltage of each of the 4 cells in the bank (and even run 2 parallel banks with just one Arduino). I'm assuming I need to monitor the discharge voltage of each cell, and monitor the charge voltage of each cell. With a shunt, I'd even be able to monitor the current going through the system.* Is that all I need the "BMS" to do? I could set up a contactor between the battery and the load/charger so that if the bms tripped due to any cell getting out of spec, it would shut down any load or charging.Charging: If I don't care how long it takes to charge the bank, can I simply hook up a 10A power supply to the 12V series string, set the voltage to 14V (3.5V per cell), and let it go? At 10A, this would be .1C, which from what I've read should be very safe for the bank. I realize it won't be fast, and I realize that I would lose some efficiency, but would I hurt the batteries? The bank would usually have at least overnight to charge, if not several days.Battery Bank Wiring:If I start with 100Ah cells, this is easy - just wire 4 cells in series, monitor the voltage at each connection, subtract to get the cell voltage, and do the logic to generate signals for LED's and the relay to shut the thing down if it gets out of spec.When I add the next bank... Is it better to put the cells in parallel, and then put the parallel'd cells in series, or is it better to put two (or more) series strings in parallel? The one key difference I see here is that if I put multiple strings in parallel, then I can put a BMS point on each battery cell.I'm trying to do this on a reasonably small budget - and trying to limit the big expenses to the batteries.thoughts / suggestions?John
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Posted by: "james@deny.org" <james@deny.org>
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