Charging rate

If your battery bank is less than ~80% state of charge, yes, is very possible to put 20% to 25% ore even more current into your flooded cell lead acid battery bank (below 80% SOC, 20% or rate of charge is just fine). Lead Acid Batteries are very efficient (little heat/gassing) when charging at lower states of charge.

AGM batteries (which I think you have?) are charging voltage sensitive (high charging voltage "gasses" AGMs which is very hard on them). But AGM batteries can also take (and supply) higher current levels than flooded cell:

http://www.solar-electric.com/lib/wind-sun/surrette-manual.pdf (page 5 and 6)

AGM Bulk Stage

The charger should deliver the initial current B until the voltage limit A is
reached. Absorption Stage–the charger should maintain the voltage A until
the current tapers to B. The initial charge current is recommended to be set at
B = 0.25 X C20 (Imax = 0.35 X C20) in order to fully charge the batteries within a
reasonable amount of time. It can be set lower, however; please be aware that
charge time will increase so make sure the batteries have enough time to fully charge
before being put back into service. Rolls batteries have a low internal resistance
allowing them to be charged at a higher current, therefore, faster than conventional
flooded/wet batteries.

With proper (temperature compensated) Absorb set point voltage, the battery bank will naturally drop below 25% rate of charge. Per battery rate of charge:

• 428 AH * 0.25 rate of charge = 107 Amps "nominal" rate of charge (per Rolls document--after a very quick read)

As always, read the documentation and talk with the vendor if you have questions.

-Bill

What Vtmaps is saying--Is that all charging sources, if powered up at the same time, their output current adds. If you have an 60 Amp solar charge controller (large enough array, full sun, etc.) and a 60 amp AC charge controller or inverter-charger (backup genset, utility power); both up and running will add together for a potential of 120 Amps of charging current.

If you run the AC genset only at night/during bad weather, then the solar charge controller won't be "on", so the 60 amps from the AC inverter-charger would be the only charging current at that time.

Regarding charging rate--I guess I goofed up... Assuming you have standard flooded cell lead acid batteries (AGM are also Lead Acid--Just different internal construction)--They have less surge/high current capabilities.

In general, we here like to recommend C/8 or 12.5% (~13%) as the maximum charging current. You can charge with higher current levels if the battery bank is less than 80% state of charge (below 80% SOC, flooded cell batteries are much more efficient and generate less heat during charging)--And a typical method folks use for charging... Wait until battery battery bank drops to ~50% of capacity, start genset early in the morning run genset until ~80% state of charge. At that point, turn off genset and let solar power finish charging (later in sunny day, or repeat next day if bad weather).

You can charge upwards of 20-25% rate of charge--But you should have a remote battery temperature sensor to keep the battery from over heating. Roughly, if the batter is over ~35C, you don't really want to charge it at high currents... And stop charging if over 50C.

Lead acid batteries are pretty rugged--But heat can kill them (as well as under/over charging, boiling dry, pulling current until battery is dead, etc.). More or less, for every 10C increase in battery temperature, they will have 1/2 the life (25C room temperature, 35C, battery ages 2x faster, 45C is 4x faster, etc.).... This aging factor is true for almost anything (from batteries, to electronics, to other materials). The aging is based on the actual temperature... The accelerated aging is only happening if the battery is at that temperature. For many folks with mountain cabins/live in the far north, they have very long battery life (i.e., battery sits at 0C for much of the time, battery last 4x longer).

This is different that cycle life--You still have that to contend with too--More/deeper cycling, the shorter the cycle life (more or less, when cycling a lead acid battery--Cycle to 80% or less state of charge--Floating/shallow cycling is not great for them either). 50% is the deepest day to day cycling typically recommended for deep cycle lead acid batteries (for longer life).

Remember too--That loads take away from battery charging current. If you are running a well pump to fill the cistern--That current does not go to charging the battery, but to running the pump instead.

-Bill

Sorry for the "fuzzy" answer... More or less, a standard lead acid battery will not (usually) over heat if it is limited to C/8 or (1/8=) 12.5% rate of charging with the correctly set Absorb voltage for the charger.

However--Cases where it is possible to over heat revolve around battery temperature, the ability of the controller to measure battery temperature, and voltage offsets based on temperature.

Because the charging voltage of a battery bank should be reduced by -5 mVolts per C per cell over ~25C, this is one way we "limit" charging current to the battery bank. However a "cheap" charge controller may not have the ability to measure room air temperature, it may supply too much current to the battery bank and cause excessive current flow (some very basic AC battery chargers are like this). Other issues may include battery in one room, controller in another room--To address that, the use of a remote battery temperature sensor attached to one of the batteries so controller does adjust charging voltage based on actual battery temperature (RBTS or BTS).

And here is the hand waving... Because the charging voltage should be reduced if the battery gets hot--If the battery gets hot without the controller "knowing this fact", as the battery heats, its voltage drops, and the controller attempts to send more current to make up for the low voltage (i.e., controller thinks battery needs more charging current / there are other DC loads in the system to be supplied). The is a "positive feed back loop"--Battery heats, controller pumps more current in, battery gets hotter, more current, You can end up with a damaged battery bank or even a fire.

Then there is what makes for a hot battery... Below 80% state of charge, you can put in C/5 (20%) rate of charge pretty easily (very little of charging current is converted to heat--Almost no gassing and very low resistance). And below 50% SOC, probably even more. When the battery is in the ~80-90% SOC, 10% to 13% maximum current--Where the maximum current is "adjusted" by the charging voltage (14.2 volts charging will still charge but at a lower voltage; 14.8 volts will charge at a pretty high current). Above 90% state of charge, even a 5% or as low as 2.5% rate of charge can overheat a battery (most of the charging current is turned into gasses and heat).

So, for lead acid batteries, one of the limits is the gassing voltage:

http://www.powerstream.com/SLA.htm

Anything above 2.15 volts per cell will charge a lead acid battery, this is the voltage of the basic chemistry. This also means than nothing below 2.15 volts per cell will do any charging (12.9V for a 12V battery) However, most of the time a higher voltage is used because it forces the charging reaction at a higher rate. Charging at the minimum voltage will take a long long time. As you increase the voltage to get faster charging, the voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place. The typical charging voltage is between 2.15 volts per cell (12.9 volts for a 6 cell battery) and 2.35 volts per cell (14.1 volts for a 6 cell battery). These voltages are appropriate to apply to a fully charged battery without overcharging or damage. If the battery is not fully charged you can use much higher voltages without damage because the charging reaction takes precedence over any over-charge chemical reactions until the battery is fully charged. This is why a battery charger can operate at 14.4 to 15 volts during the bulk-charge phase of the charge cycle.

The basic lead acid battery is ancient and a lot of different charge methods have been used. In the old days, when voltage was difficult to regulate accurately flooded lead acid batteries were important because the water can be replaced. The lead acid chemistry is fairly tolerant of overcharging, which allows marketing organizations to get to extremely cheap chargers, even sealed lead acid batteries can recycle the gasses produced to prevent damage to the battery as long as the charge rate is slow. We offer a range of chargers from inexpensive to very sophisticated, depending on the requirements of the customer, but all of the chargers we sell off-the-shelf are highly regulated sophisticated chargers that cannot overcharge the battery.

Notice that if the battery is not fully charged, the charging chemistry takes precedence over the "gassing chemistry" And since we have a limited amount of hours of sun during the day, we try to recharge the battery bank as fast as we can without damage (thermal damage and electrical/physical damage). If all things were equal,you would get longer battery life if you gently charged at 10% or so rate of charge for 8-10+ hours per day (from 50% state of charge) at a lower charging voltage. However, since there are limited hours of sun per day, we jack up the charging voltage/current to get >80% SOC quickly--Plus we may have other loads during the day (washing machine, run the vacuum, well pump, etc.). So--To meet the minimum 10% rate of charge + supply additional loads that may be present, we want to have >~10% rate of charge (which can overheat the battery bank--vicious circle).

Midnite now has the Wizbang Jr. system (battery current shunt) so it can measure actual charging current. You can have a >>13% rate of charge available from charging source, but the solar charge controller + Wizbang Jr. will limit actual charging current to the battery to the 10% or 13% (you program--Of course this only limits the solar MPPT charge controller, if you have a 20% AC Battery charger, the Wizbang Jr. cannot control that current source).

Anyway, lots of hand waving--Does this help?

-Bill