Lithium Phosphate Batteries

Wanderman
Wanderman Solar Expert Posts: 180 ✭✭✭
Hey All,
Lately I have come upon a small snag in my quest for an off grid RV solar solution.

In a nutshell, I've sourced 3 36v nominal panels from a manufacturer in China. They are custom sized to fit my roof, which is quite small. They produce ~450w at ~36v and feed a Morningstar MPPT-45amp controller.

The original battery bank is/was 2 Trojan SLA 27TMX batteries with 105a/hrs each.

The plan was to switch to 3 Trojan SLA SCS225 batteries with 130 a/hrs each.

Since we've had some better weather, I went out an re-measured the existing battery box. It seems to fit these 3 batteries (20 1/4" x 13 15/16" x 9 7/8") I will have to re-design the entire drawer assembly. It's 17 3/8" x 16" x 10 7/8".
I'd rather not have to do this.

2 batteries Will fit OK but only give me 2x 130a/hr capacity.

I did some research online and found there are Lithium Phosphate batteries available as replacements for SLAs

http://lithiumion-batteries.com/RVDEEPCYCLE.aspx

There is at least one test customer that has both chassis and house batteries fitted.

http://www.aboutrving.com/rr_one.php

Any comments on this? I can find many custom configured 4 x 3.2 cells to give a relatively flat 12.8v profile. They appear to be able to charge faster and be discharged further down than an SLA. I do see a small gotcha if the cells are permitted to get below 2.2v each they are toast and must be tossed. According to the user article they seem to have modified (slightly) the set-points of their charger to meet the Lithium Phosphate specs.

I may have a problem with my PD-4645 converter charger it has 4 stages:

BOOST Mode 14.4 Volts

NORMAL Mode 13.6 Volts

STORAGE Mode 13.2 Volts

DESULFATION Mode 14.4 Volts every 21 hours for a period of 15 minutes to prevent battery stratification.

With the exception of cost, this seems like an ideal replacement with higher energy density and MUCH lighter weights.

I am open to other solutions.

Thanks,

Rick

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: Lithium Phosphate Batteries

    Nice thread here:
    Lithium Iron Phosphate Batteries; LFP; LiFePO4 discussions

    These batteries sound pretty nice.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Lithium Phosphate Batteries

    wow, maybe these are finally coming mainstream, they are getting a call from me, and maybe FLA battery prices will drop.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Lithium Phosphate Batteries

    OK, I dig the weight to power, but did you guys miss the BIG RED NUMBERS?

    We're talking $2,500+ for a 12v 200AH battery or a little more than what my 700AH 24V fork lift battery will cost delivered!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Lithium Phosphate Batteries

    LiFePO4 batteries are a great choice for off grid. They are perfectly happy with continual partial charge state and have >90% charging efficiency.

    The main issue is they don't take to overcharging. This is a problem with cell balancing in a series string of 8 or 16 cells for a 24vdc or 48 vdc system, respectively. If you use them without a charge rebalancer there will be eventual cell damage as the batteries diverge in their SOC condition.

    For a large AH off grid setup, shunt resistive balancing is not practical because the shunts would have large power dissipation of 100 to 300 watts.

    You need an active switcher power supply rebalancing system which is quite complicated and expensive. There are systems used in automotive that have bidirectional charger/loads connected to each cell to rebalance charge and discharging. I believe for off grid applications you can get away with a charge current shunt active switching power supply that pumps 3.6v @ 30 to 50 amps to top of stack voltage to suck away charging current when cell approaches full charge. You can probably get away with a boost switcher count of something like a third of the battery count and have the processor switch them around as needed to balance the charge topping off process. Down side of this is you would need wiring to each cell capable of 30 to 50 amps to draw off the excess charge current. Quite a wiring rats nest.

    It would probably be more practical to have a 100 watt to 200 watt switcher power booster module on each cell that pushes excess charge current to the top stack voltage, returning the excess current to 24v/48v main charging input. This would still require a common bus to pick up the excess current from all the modules although it doesn't have to carry as large a current as the individual high current draw approach.

    You also need the individual modules to output a shut down signal to the inverter if any of the cells reach there low voltage discharge limit.

    Solve the charging issue and they are a great solution for off grid power source.

    On the cost side, they have the potential of being cost effective against lead-acid batteries if they achieve their 2000-3000 full cycle lifespan. Adding in the considerable but potentially declining cost of the battery management system levels the cost benefit. It would be nice to get 20-30 years out of a set of batteries with >90% recharging efficiency.
  • Wanderman
    Wanderman Solar Expert Posts: 180 ✭✭✭
    Re: Lithium Phosphate Batteries

    OK. so here are some reasonably priced 200a/hr Lithium Phosphate cells:

    http://www.thunder-sky.com/pdf/2010723142434.pdf

    I know about the iffy reputation of the Thundersky brand, but now they are called Winston Battery Limited and have a small, but growing dealer network to warranty and distribute their products.

    These cells,

    MODEL NO:WB-LYP200AHA(B)
    Nominal Capacity :200AH
    Operating Voltage:2.8V~4.0V
    weight:7.3kg±100g
    Dimensions:362×55.5×256(mm)
    14.25" x 2.2"x 9.9"

    I could fit 8 of these ~3.2V cells in the space. Wired series/parallel that would give me 200a/hr x 2 12v groups totaling 400a/hr. Discharging to 80% gives me 320 a/hr usable and ~2000 cycles (maybe 3000) to 70% gives me 280 a/hrs usable.

    Likely more than I need. But extra capacity is OK.

    Doing the math.

    A typical 130a/hr Trojan (SCS225) costs around 190.00 or so.

    These cells cost ~240 each.

    Total Capacity for Lead Acid:
    3 batteries = 390a/hr @50% discharge =195 a/hr =$570 * (~500-1000cycles)

    Total Capacity for Lithium Phosphate:
    8 cells = 400 a/hr @80%(70%) = 320a/hr (280a/hr) =$1920 * (~2000-3000cycles)

    Looking at cycles only the cost ranges from $1140 for Lead Acid or $1710 vs $1920 for Lithium Phosphate over the same lifetime.

    If you look at capacity matching you could almost double the Lead Acid costs.

    Weight savings is quite good as well with Lead Acid at 66lbs per battery and Lithium Phosphate at 64lbs per 12v series. Based on capacity.

    My biggest worry is proper charging and use. These cells range from 2.8v - 4.0v giving 11.2v- 16v range. 16v may be a bit high for 12 volt use. I would cut them off from the circuit if voltage goes below a set (guaranteed no damage level)

    This looks promising.

    Rick
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Li batteries of any style, all must stop recharge at 32F and below. 
     Solutions are
    an insulated, heated battery box ( recharge efficiency is so high, there is little self-heating)
    batteries with a BMS and internal heater that manages temperature.
    Avoid frosty areas

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,