Flooded Lead Acid Battery Management Wall Chart

STANDARD RESTING CHARGES, 2 VOLT CELLS, SLI & Deep Cycle @ 25C (.01-.05v FC vpc difference is OK, all vpcs drift over time, and cells will trickle discharge also, when not in use)

SOC % 12 VOLT 24 VOLT VPC max 2.583vpc @15v and 30v resp.
100 12.70 25.40 2.12/5  
90 12.50 25.00 2.08 20% SLI battery cycle maximum
80 12.42 24.84 2.07  
70 12.32 24.64 2.05  
60 12.20 24.40 2.03 40% solar battery optimum DOD
50 12.06 24.12 2.01 50%
40 11.90 23.80 1.98   RULES of THUMB:
30 11.75 23.50 1.96   SG+0845= vpc, resting charge
20 11.58 23.16 1.93 80% Ohms pc = 7.5, full resting charge
10 11.31 22.62 1.89   (both @ 25C, normal acid range)
0 10.51 21.02 1.75 Full DOD

24 VOLT CHARGING STATES: Do not discharge below 20-40% resting charge if possible. Battery (bank) will charge to 25.4v+ by PV or generator, (each day if possible), but for optimum battery health, a proper charging regime should follow, min. 2 hours per month. Equalize when there is a min of .2v difference between cells, regardless, smaller batteries will require less electrolyte stirring. Routine charging should extend into absorption @ 25C and above, up to 28.6v; and beyond absorption, up to 29.6v, when below 12.7C. (NB, for 12v, halve 24v values)

Boost @ 20 amps to 30v, briefly, full charge @30v is 2.583 vpc, including
Absorption 27.2v to 28.4v, charge rate may slow slightly. Then for
Equalisation take to 31v, taper amps to maintain venting 30-31v, 30 to 180 mins.
Float Drop amps, taper to 25.6v-26.4v, maintain 2.15-23 vpc.
Rest Being fully charged 25.4v, @ 2.12-15vpc, EMF

IN DETAIL: Advance charge @ <20 amps, and raise only when needed to initiate equalization at 31 volts, and for de-sulphation, when amps are then tapered, to enable vpc of 2.4/5v. Both may be concurrent. Bubbling and venting at 30-31v will be sufficient for equalization. Bubbling begins at 2.2vpc, so beware of overcharging beyond 31v, damaging and unnecessary, causing heat and plate breakage. (An electronic desulphator is recommended?)


good SLI 1275 to 1300
good  DC  1220 to 1240  
Test for SG peak during equalisation, then discontinue charging; add or subtract .004 for
satis   "    1225 to1250
satis      "  1200 to 1220
  each 5C above or below 25C for SG correction.
poor   "    1150 to1200
poor      "  1100 to 1150

Voltage and hydrometer readings should always be concurrently optimum. Battery capacity is progressively limited with cold, so factor this into winter usage calculations. Freezing point means 35% capacity reduction, plus the limitations caused by age and condition.  Heat and/or poor plate condition cause resistance, and cold limits capacity.
Battery temperature and state will influence charging, by PV or generator. Battery life is halved by every 9C over 25C, if unregulated.
Capacity to receive charge v temp: F/C & good SG: @ 27C, 100%; @ 0C, 65%; @-18C, 40% Temperature regulation for battery banks is strongly recommended.

The quicker and higher the charge return and hold, the healthier the battery.  Regular top-ups and/or trickle charging are needed when batteries are not in use, see FLOAT above, and the normal charge routine still applies. Always use 4-stage regulators to taper any charge, especially for unattended operations.

Slow and steady C10 charge and C20 discharge best, 20-40% discharge only, for longer battery life, no large quick drains or shallow cycles under 5%, and use a generator for high demand periods, for PV charge cycle lag and/or equalisation.
Allow adequate time for plate reformation, the key to optimum battery charging and long-term health.  When fully charged, the anode, PbO2, should be noticeably chocolate in colour, and the lead  cathode, Pb, should be grey.

Battery temp 25C for optimum performance, 25C max for PV panels. A brewstrip thermometer on the battery, plus calibrating wall thermometer, account for temperature lag.  Ensure adequate ventilation of batteries at all times. Rotate end battery positions minimum once per annum, ad hoc, to maintain uniform condition. 

Make a chart to track individual battery health, record readings regularly.  Check connections and wiring first when faultfinding, before more complex steps!

Customise these Wall Chart figures according to ambient temperatures, manufacturers specifications, plate chemistry, and/or individual PV systems, when necessary.  AGM and Gel VRLA battery maximum charge input is only up to 2.35 vpc, use dedicated VRLA regulators!

Free PDF and WORD versions, PDF of this Wall Chart, and of the 72 page booklet, Home Solar Power Management, and Why are availble at the Home Page:

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