UPDATE- Solar PV's 20 (6V) batteries

Re: UPDATE- Solar PV's 20 (6V) batteries

Kevin,

The discharge rate specs provide an indication of total battery capacity based on certain time and/or load conditions. The common spec is 1.75 Volts per cell (Vpc), which is the voltage of a fully discharged single battery cell under load at 77 degrees F. For a typical 12 V battery with six cells, this spec says that the battery is fully discharged when it reaches 10.5 V under load.

The “8 hr, 25 A” spec means that the healthy and fully-charged battery can sustain a 25 A load for 8 hours at 77 F until the single cell voltage drops to 1.75 Vpc, or 5.25 V for a nominal 6 V battery. This suggests a total battery capacity of 25 A x 8 hours = 200 Ah.

The common standard battery capacity spec is based on a 20 hour discharge period. Based on the specs provides, I would estimated the 20 hours capacity of each battery to be ~250 Ah, or 12.5 A x 20 hours.

As a practical matter, fully discharging a battery greatly reduces its useful life. Typical average daily discharge is in the 10% to 20% range, and batteries are rarely discharged below 50% stage of charge (SOC).

Accordingly, the actual nominal total capacity of your 24 V battery bank is likely ~1,000 Ah, cased on the estimated 20 hours spec. However, typical daily discharge should only be in the 100 Ah to 200 Ah range, and you avoid taking more than 500 Ah from the batteries before recharging them.

Flooded-cell deep-cycle batteries require a four-stage charger for proper charging and maintenance: bulk, absorption, float, and equalization. For a 24 V system, the target voltage for the latter three stages should be ~29.6 V, 26.4 V, and ~31 V, all at ~77 F. Cold batteries require higher voltages, and warm/hot batteries require lower voltages. The resting voltage (no loads or charging for at least three hours) for your "24 V" battery bank will be ~25.4 V.


Automotive/truck/dozer alternators are spec’d to recharge starting (SLI) batteries, not deep-cycle batteries. Accordingly, I doubt that your dozer alternator will be able to properly charge the battery bank.

Trojan Battery recommends a charge rated at ~10% to 13% of battery capacity. For your 1,000 Ah battery bank, that would be something in the 100 A to 130 A range, at 24 V to 31 V nominal.

You should use a true sine wave inverter. Your power tools and other motorized loads will likely run cooler and last longer. The size of the inverter (on Watts) will depend on your loads.

One very good and rugged inverter to consider is the OutBack VFX3524. It can deliver 3500 W continuously, and it includes an 85 A four-stage charger that can powered from a generator. Although <100 A, the 85 A charger is big enough for the 1,000 Ah battery bank. It ain't cheap, but it's worth the price.



You’d need an 120 VAC generator rated at ~3,600 W continuous, a 3,600 W (continuous) 120/240 V generator with a balancing transformer, or an ~7,200 W (continuous) 120/240 V generator to run the 3524’s charger.

A battery monitor is required to monitor a battery bank’s SOC. The Xantrex Link-10 is a good choice.

HTH,
Jim / crewzer