What constitutes a cycle for the battery bank?

[larry2c]

I recently installed a solar system on our TT and from what I read, most people that have solar on their RV leave it continuously charge their battery bank. However, I have read that most flooded batteries are good for about 1000 cycles (charge/discharge/charge). If I was looking at normal use cycles, this would mean our batteries would last a really long time.

Then I got to thinking - every night there is a slight parasitic load on the batteries and the next day when the sun comes up, the CC detects that the battery could use topped off so it goes into charge mode (bulk mode for a bit and then float mode). Of course this happens every day so in less than 3 years you'd have 1000 cycles.

So do these "mini" cycles count? If the TT is going to sit for a few weeks should I be flipping the disconnect for the panels (cuts input to CC)? Maybe I'm overthinking this but I figured I'd ask...

[TitanMike]

I would not worry about the mini-cycles. If you look at all the charts that show battery life versus depth of discharge percentage, the depth of discharge is a bigger factor in shortening the battery life.

One thing that occurs regardless of depth of discharge or charging is corrosion of internal battery components. Age will eventually take it's toll due to this.

I leave my TT plugged in whenever it's home. Any parasitic current draw or internal discharge is overcome by the converter/charger.

I figure a Lead Acid Battery is only going to give me max performance for 3-4 years anyway. If I camp for 180 days per year (6 mo's) 4 years is over 700 "Deep Cycles", assuming all those days are boondocking days. Of course this will also depend greatly on the quality of the battery to begin with.

I wouldn't worry about "mini-cycles". Just maintain electrolyte levels, keep terminals clean, and make sure you keep your depth of discharge at 50% or Less. Batteries will last several years. A good battery monitor will help you get as much life as possible out of them by more accurately monitoring depth of discharge and telling you when they are fully charged (both by percent of charrge and accurate voltage measurement).

[formerFR]

'cycles' is just a way to 'approximate' or generalize the ability to compare the output life or longevity of a specific battery type... the 'cycle' itself is generally referring to a 50% discharge per 'cycle', which is actually relatively RARE with most users, unless you constantly DEEPLY discharge your battery(s)...

a solar power or trickle charger or converter giving your battery(s) constant charge is not an issue since it's already 'off setting' any slight discharge.
While 'BULK' charge is what you may be thinking of, chances are your solar or charger is only going to be in 'float' charge while not in use, meaning that it is simply providing any small trickle charge, as needed. Even when you are using the RV, the solar or charger may only by in 'absord' mode, meaning that it's mostly just off-setting your usage, while also providing constant trickle charge.

[pgandw]

The cycle rating of a battery is usually based on recharging from 50% SOC. 50% is an arbitrary number chosen to get a reasonable battery capacity and number of cycles.

In real life, the SOC curve vs number of cycles is not linear. Most of the flooded golf cart battery curves I have seen show something like:

70% SOC - 1900 cycles
50% SOC - 800 cycles
40% SOC - 400 cycles
30% SOC - 110 cycles
20% SOC - 45 cycles

As you can see, the curve falls off rapidly below 50% SOC. Remember, each cycle returns ALMOST all of the capacity of the cycle before. IIRC, at the rated number of cycles, the battery is down to 50% of its new capacity.

Besides cycles, other things that limit battery life (cause a battery to lose fully charged capacity) are:

- letting water level get below covering plates at any time. This causes the uncovered plate to oxidize, and is no longer available to exchange ions with the acid.

- plate shedding. Lead falls off the plate and settles at the bottom of the cell. Caused by vibration and overcharging (too high a voltage for too long). When lead buildup touches 2 adjacent plates, cell is now shorted. In the meantime, less lead on the plates to exchange ions.

- deep discharges cause a hardened form of lead sulfate to form on plates. The hardened sulfate will not be dissolved at normal charging voltages, resulting in less lead plate for ion exchange, and lower capacity.

If I treat my golf cart batteries decently (keep the correct water level, recharge at or before discharging to 50%), I expect to get 5-6 years before significant capacity is gone, and 7-8 years is a bonus. This is based on camping 40 nights/year at most, fully charging then shutting off the batteries (complete shutoff) while at home between camping trips, recharging batteries before each trip.

Fred W
2019 Flagstaff T21TBHW A-frame w/ dual Costco/Interstate GC-2 6V batteries, PD 4135 converter.

[larry2c]

Thanks all, I appreciate the comprehensive feedback & info.

I will be keeping a close eye on the fluid level in the batteries more so now than I did in the past. When I was asking some questions a few weeks back while preparing to buy & install our solar system, one of the responses I got had a line that stuck with me "solar will relentlessly charge your batteries". Can't remember who to credit with that statement but it makes perfect sense and means you really do need to keep an eye on maintenance of the batteries.