dual battery banks

[lyonsden]

Hi everyone. I'm living off grid and am upgrading my battery bank from some old large format Nickel metal Hydride cells made by Panasonic many years ago to some LiFePO4 batteries. I'm starting out small and planning to build over time, so I am for now using both battery banks with a dual battery switch. I split off after my two charge controllers to both battery banks, but noticed that the new lifepo bank is maintaining the nickle bank when they are not being charged. The new bank is obviously feeding back to the old bank and keeping it charged at night. Is this a bad thing, and should I put a shunt or diode in the charge line to the new lifepo bank to keep it from maintaining the old nickle bank? I was shocked to wake up with over 13v left on the nickle bank in the morning for the first time ever. It's an interesting concept using a small high end lifepo bank to maintain an old tired battery bank, but is there a problem or disadvantage to this. I appreciate any advise for someone who is new to this scenario.

[Mike90250]

if a bank is old and tired, I'd not waste time and power keeping it alive, recycle it, sell it, fund your new bank with it.

I see little benefit to have two small banks and always be flip flopping between them, just size the bank properly and use it.

[Ampster]

............... I split off after my two charge controllers to both battery banks, but noticed that the new lifepo bank is maintaining the nickle bank when they are not being charged. The new bank is obviously feeding back to the old bank and keeping it charged at night. Is this a bad thing, and should I put a shunt or diode in the charge line to the new lifepo bank to keep it from maintaining the old nickle bank? I was shocked to wake up with over 13v left on the nickle bank in the morning for the first time ever. It's an interesting concept using a small high end lifepo bank to maintain an old tired battery bank, but is there a problem or disadvantage to this. I appreciate any advise for someone who is new to this scenario.

Tell me more about your LFP battery? Is it 4 cells? Voltage flow in parallel packs is from high voltage to low until they equal voltage. I would guess the two chemistries have different resting voltages. If you took 4 LFPs to 13v that is 3.25 per LFP cell. I agree with Mike90250, that there is little benefit having two banks, especially if they are different chemistries. You are discharging the LFP to maintain the NMH. Is there really enough Watthours left in the NMH that you want to put the LFP through more cycles than needed?

[lyonsden]

I'm sure you guys are right. The only reason to do what I'm doing is money. As you know, liFePO4 batteries are pretty pricey, and I just finished building on off grid house, which was also quite pricey. My plan was to build the bank over the next 6 months or so as I can afford, while utilizing the lithium I have while utilizing the NiMH batteries at the same time. I'm not exactly a hoarder, but still hate getting rid of something that still works, which these batteries do. Unfortunately, it is in there nature when they age to self discharge at a high rate, and a BMS for NiMH is more expensive than the modules themselves. Also no one recycles them around here, and they are pricey to ship. It's easy enough to switch back and forth.
My LIfepo4 batteries thus far consist of 16 3.2v 100Ah cells in a 4S4P configuration. That's four in parallel to give me four 400Ah 3.2v cells. Then four of those wired in series to give me my 12.8 v nominal voltage. anyway , everything works well, but it would be nice to separate them completely via the line from the charge controller so that the lithium are not maintaining the NiMH bank. Seems like it should be an easy thing, but I want to do it right.

[littleharbor]

Your long range plan is to power your off grid home with a 12 volt based system?

[lyonsden]

when I eventually have enough batteries I will also upgrade my inverter to 48v, I designed my home to be very energy efficient. I have no 240v, only 120, and I use very little energy. I have a rainwater catch system with an indoor cistern, and some 12v dc appliances that do not run through my inverter. I could easily live with the 12v system indefinitely, but because of efficiency I will eventually upgrade to 48v.

[littleharbor]

when I eventually have enough batteries I will also upgrade my inverter to 48v, I designed my home to be very energy efficient. I have no 240v, only 120, and I use very little energy. I have a rainwater catch system with an indoor cistern, and some 12v dc appliances that do not run through my inverter. I could easily live with the 12v system indefinitely, but because of efficiency I will eventually upgrade to 48v.

Good answer. Start with a high quality charge controller and you may never have to replace it because of the ability to quadruple your array wattage at 48 volts

[lyonsden]

Thanks for all of your help and advice guys, but I still don't seem to be able to justify getting rid of my NiMh battery bank. Especially now, since I have done some testing of my own and eliminated the trouble makers, and now have a good bank of NiMh batteries. I still plan to upgrade and have already started to do so, and am using the NiMh bank solely for my 12v dc panel, and am using the Lifepo4 bank for my inverter. It is working very well , and don't see the need to junk the good NiMh cells. I would like to put some sort of component between the two to stop the charge to the NiMh bank while the LIfepo4 bank continues to charge. The NiMh batteries don't like it over 13.4 v or so, which is a full volt lower than the lifepo4 bank. Is there an economical component which will read and stop the charge coming in to the NIMh bank when it has reached it's peak voltage. I see some over voltage protection circuit boards for sale on line, but am not sure if that is what I need, or if a voltage regulator would do the same thing? They are both charging from the same two (two separate arrays) MPPT charge controllers. It works great the way it is, but I'm sure that I will eventually trash the NiMh cells charging them to 14v. So far I've programmed my controllers to undercharge my bank until I get my BMS. Once installed, I will up my charge to optimize the lifepo4 bank, which won't be good news for the NiMh bank. I may wind up getting rid of them after all, but It would be nice to keep it as it is if I could find a way to limit the charge to the NiMh cells.

[inetdog]

NiMh cells can be very particular about charging conditions and detecting end of charge when trying to fast charge close to 100%. The failure modes can be quite bad! If you have a charger which is specifically designed for NiMh, I would not as concerned, but if you are trying to cobble something together with another battery bank as the source, I would not recommend it.

[Mike90250]

A simple series diode will drop 1/2 volt, so 2 high amp diodes in series ought to help. What's the Ah of the bank, and what's the charging Amps your source can provide ??

[inetdog]

lyonsden:

Please read the section on charging NiMh batteries at BatteryUniversity!

https://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

Trickle/Float charging should be done at a current of .05C or less. That means that the Float charge current must be limited to the 1/20 of full capacity per hour. Too slow to be particularly useful for actual recharging in most cases. To do this from another battery bank without using an active charging circuit requires a suitable series resistor to limit current, not just a voltage control.
Faster charging of NiMh requires the detection of full charge in the form of a DROP in the cell voltage under charge of about 5mV. For faster charging it is also critical to be able to monitor the battery temperature while charging.

If all you need is to float the batteries and never apply any load to them requiring recharge, then by all means go ahead. Otherwise I seriously recommend that you not try the suggestions of Mike or other usually reliable people.

[lyonsden]

Thanks a lot for the info and the link. I'll read it tonight. Yeah, the nimh was probably not the best choice, but I have to say, they have worked great since I've been using them. Also the cells I have are probably the largest format NiMh cells made, and are not subject to some of the drawbacks that their smaller counterparts suffer from. I know the charging algorithm is more complex, making them a pain to charge. A BMS for these cells cost more than the cells themselves. They are being charged with a good MPPT charge controller, but it looks like their days are numbered as I will eliminate them completely once I get the rest of my lifepo4 cells soon. Because these cells were remanufactured I never knew the actual capacity, and the high self discharge rate makes that difficult to determine. I got them because of their compact size and their ability to fit in my small space. Unfortunately, the remanufacturing process did not take into account the self discharge rate, which eventually prompted me to eliminate nearly half of my batteries. Lesson learned. At least they weren't too expensive. Now I have over 100 cells to get rid of. At least the ones I have left are working well, and will be fine until I complete my new system. Thanks again for your advice.