New Li Ion Batteries to replace my Lead Acid--What next?

RobWaugh

I've just got 8x 3.7v lithium ion batteries to replace my old lead acid ones, a straight swap,I'm completely off grid and want to know do I just do a straight swap or do I need some sort of new fangled electronic gizmo, also what should I set my charger controller to for maximum and minimum volts ?

tia Rob

BB.

I will start off--But there are others here that know way more than I on the subject.

First, read the battery manual and make sure you have programmed the controller to not over charge the batteries. And generally LiFePO4 batteries do not have any temperature compensation (lead acid batteries, the charging voltage rises as the battery bank gets colder).

Li Ion batteries tend to last longer if you keep them somewhere in the range of 20% to 90% state of charge. Set your charger to charge to 90% and your Low Voltage Warning/Disconnect to ~20% of load. With battery balancing (see next), this helps to ensure that each cell (parallel groups of cells) will not be over or under voltaged.

Measure the voltage of each cell (group of each parallel cells). They should be very close to each other (millivolt level). Charge or discharge cells to get them "matched" voltage. For electric vehicles, they typically "bottom balance" the cells (they match as the cells approach zero state of charge). For off grid power, typically "top balance" the cells--To prevent one cell that is "higher voltage" from being overcharged and damaged as the bank gets fully charged.

Nominally, once the battery bank has been properly balanced, Li Ion cells tend to stay in balance. In general, because the cells are "eash to damage" if overcharged or over discharged, many folks install a battery management system (BMS). Some just monitor the cell voltages, others can actually (slowly) balance the cells. BMS can also control overall charging/discharging with the ability to disconnect the battery bank as a fail safe.

And (I am not sure lastly), make sure that you have some sort of fuse(s) or circuit breaker(s) on your positive bus... Li Ion batteries can supply very high current into short circuits. You need to size the wiring and breakers to the maximum continuous load(s) and charging current you expect. I personally suggest that you multiply your maximum continuous current by 1.25 (NEC derating) and size the wire and fuses/breakers appropriately (remember the fuses and breakers are there to protect the wiring from catching fire, not the attached devices).

Li Ion battery fires while rare (and LiFePO4 are among the safest Li chemistries), keeping the cells within the working voltage range is critical. Also, if possible, placing the batteries in a fire resistant 'out building' or exterior box can help isolate damage from affecting your home (and Li batteries tend to just burn themselves out with water applied to lessen the spread of fire. And they can release various forms of Hydrofluoric Acid from a fire--Very dangerous stuff--Don't breath or get on your skin).

-Bill

Estragon

8 x 3.7v (29.6 | 14.8?) seems odd to me. Voltage might be too high for common devices? AFAIK, lifepo are lower nominal voltage (~3.2v), so I assume these are some other lithium chemistry?

mcgivor

@RobWaugh said

I've just got 8x 3.7v lithium ion batteries to replace my old lead acid ones, a straight swap,I'm completely off grid and want to know do I just do a straight swap or do I need some sort of new fangled electronic gizmo, also what should I set my charger controller to for maximum and minimum volts ?

tia R

Regardless of actual chemistry, it is not just a matter of conneecting in series and replacing a lead acid bank. Lithium batteries are very sensitive to voltage especially in the upper and lower extremes, so it's important to understand what the setpoints are. Without knowing specifics it's not possible to recommend anything, other than general cautions.

Could more details be supplied, battery type, make of controller, amp hour capacity and any other relevant information which may be useful.

Tecnodave

At 3.7 volts these are not LiFePO4 cells...they are LiLo or LiPo cells or LiPo pouches. LiFePo4 cells are damaged at 3.7 volts and have a nominal voltage of 3.2 volts with a normal maximum charge voltage of 3.5 or 3.6 volts. There is lots to learn about lithium batteries and they are not a ready swap in for lead acid. I have been reading about lithium about 2 years now and have a 13 volt set of CALB 40 a.h. LiFePO4 cells and about 60-70 lilo 18650 cells and a dozen LiPo pouches in use. I have done bottom balancing to match mismatched cells and I do have a balance charger with built in BMS. Even though I have done lead acid for many years there is much to learn when switching to this technology. I have seen eBay battery sets made up of mismatched 18650 rejects that were rewrapped to look good but when disassembled to check each cell, many were not usable. I would not trust these in any critical application as they will fail. Massively paralled small cells are far more prone to failure of a single cell which leads to a chain reaction. Large format Prisimatic cells such as the CALB (China aviation lithium battery). cells or BYD (build your dreams) cells are the best for solar use. I would avoid the 18650 type as too complicated for this use and far more prone to massive failure.

david

Tecnodave

I have found some of the best info on lithium types on the radio remote model airplane groups where they use LiPo pouches due to their high power to weight ratio, and on the home brew electric car forums where LiFePO4 Prisimatic cells are the form factor of choice.

companies I have personally dealt with and have very knowledgable staff are:

ProgressiveRC.com Model airplane guys , large selection of high quality LiLo and LiPo cells and chargers

Junsi Manufacturer of the iCharger line of lithium charger......I have two iCharger i208B chargers/balancers

electriccarpartscompany Specialist in home brew electric car parts and good prices on LiFePO4 cells from CALB and others

there are others but these were the best that I have found yet.

I am building an e-bike which will use 50-90 used 18650 cells removed from outdated laptop batteries and Chinese hub motors.

i have found that computer manufacturers use their 18650 lithium packs very conservately and have a 95% rate of recovery of used cells from older laptops. I gutted 100+ H-P laptops for the batteries and all cells were good. H-P uses Sanyo and Panasonic cells, most are 18650 form factor but I have some LiPo pouches that were removed from Acer net books. These are a waste product that most recycle centers have to pay for disposal of. I agree to properly recycle the non usable ones and get them free from various recyclers. I return dead or questionable ones and all the plastics for recycling and they are happy with that. Power tool batteries are used much harder and I have a lower rate of recovery with them . I have successfully repowered my older power tools that had ni-cad of ni-mh batteries with 18650's from the laptops. I hack open the battery cases and wire in the 18650 cells and a cell balance connector which plugs into my iCharger balance charger (BMS). These are charged from my 24 volt solar system. The Junsi iChargers are buck-boost converters which are DC in DC out and are bi-directional. Yes you can charge your lithium set from the lead acid set and discharge them back into the source battery. Some lithium so should not be stored at full charge but need to be discharged down to a storage level. LiPo pouches are in this category.

There is lots to learn about batteries, I'm still learning this technology.

davi

RobWaugh

Thanks everyone for the information , I kinda got court out because my new batteries arrived a lot faster than expected, I've been desperately searching the internet for the meanings of the abbreviations for components.

thanks again, I'm going to sit in a darkened room and try to figure it all out😬

Rob