Newbie From Down Under - New Zealand

Re: Newbie From Down Under - New Zealand

The issues with discharging too deeply for any rechargeable battery is the issue of taking one cell "dead" while the others still have available charge (unmatched cells can vary in capacity by 10% to even as much as 20%)... The once cell goes to zero volts and even begins to "reverse charge"--Which is a killer for virtually any battery chemistry.

And taking a battery down to 20% SOC is always an issue in load management--And even knowing that you are at 20% SOC and not 30% or 10% SOC instead. Lead acid batteries will typically lose a lot of life if taken that low (and non-deep cycle type batteries will be toast if taken this low).

Generally, our rules of thumbs are 1-3 days of storage and 50% maximum discharge. So, for a nominal system design 2 days of storage and 50% maximum discharge would point to a battery bank being ~4x larger than your daily load. You typically want to cycle deeper than 10% (lead acid batteries do better if cycled) and it gives enough battery bank size to support heavy surge currents (like starting a well pump). Also, lead acid batteries tend to sulfate faster the deeper discharged they are and the longer they remain discharged. So you don't want a very large battery bank to slowly discharge over 7 days of bad weather before recharging--That type of service will knock years off the life of a Lead Acid battery bank.

For AGM and Lithium type batteries--They can supply tremendous amounts of surge currents (some AGMs upwards of C*4 vs a typical deep cycle flooded cell of C/2.5 recommendation). But you still need AH capacity to supply those 2 days of "no sun" loads--So unless you have loads with high surge/low average power draw (HAM transmitter, well pump, etc.)--You still need a fairly good sized battery bank to manage the base loads too.

For RV type applications (and seasonal/vacation cabins) where weight and size are critical and you have a backup genset anyway--AGM or Lithium batteries can be much smaller/lighter and can cycle deeper for many users (i.e., weekend use--50 cycles a year, 500 cycle life is 10 years--And batteries will probably "age out" faster than that). Small AH capacity battery banks can be just fine for the application.

Otherwise, we still try to suggest battery systems that are not "over sized" for the application... This can push solar array size requirements way up (to meet 5-13% rate of charge). And when it comes time to replace the battery bank--You have to spend a lot of money to replace. Also, if there is an "oh heck" moment (kids/guests leave lights on when leaving for the season) and kill the battery bank--You have more money "at risk".

In the end, as vtMaps asks--Learn your load requirements and design your system around those loads (after you have spent your money and energy on conservation--it is almost always cheaper to conserve than to generate extra power). Towards that end, we usually suggest using cheaper batteries at first (training batteries). It allows you to see what your real loads are and get everyone trained. Then a few years down the road, when the first set eventually dies, you can size the new battery bank to your now known needs.

Having a Battery Monitor (like you do) will help you a lot to better understand how all this works together.

Keep us up to date on your Lithium system... You are using a standard solar battery charging system. What sort of battery/cell balancing system are you using? Any issues with over charging yet? How much of your power loads are supported by the solar array vs the other charging sources? Those can be a lot of loads for just a 330 watt array.

-Bill