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Words of wisdom

“Three stage chargers are easier on your batteries, charge them faster, fuller and help you use less water”1) – HandyBob
A [ shore power ] charger is a stiff source with unlimited energy and time. Your solar is a very soft source of unknown power and only a few hours to get the job done. - sunking2)
Charging lead acid batteries is not something that you decide to do, or start or stop. The old statement, “I need to idle the engine for a few minutes to “top off” the batteries.” is a prescription for turning expensive batteries into paper weights. You want a system that puts the batteries on charge automatically, every time there is even a single ray of sun or that your engine is running – DiploStrat3)

Charging Deep Cycle Batteries

Solar charging setups cannot charge at night so the most extreme Depth of Discharge typically occurs just before sunrise. The battery bank must be brought to full charge as soon as possible, both to enhance usability and the health of the batteries. Full charge means charging to the battery manufacturer's specification. It is not a guessing (or hoping) game.

A combination of high current charging (shore power? alternator?) combined with solar can be ideal:

“High amps in the morning when most depleted, and enough solar (hopefully more than enough) to reach and hold absorption voltage all afternoon = happy long lived batteries” – Sternwake4)

In general, manufacturers of flooded lead chemistries specify charging at C/10 to C/5; this assumes more charging time than solar power allows.5) AGM are typically charged at C/5 to C/3. There may not be a practical maximum charging rate for cycled FLA batteries when charged from solar. As Sternwake put it when describing a C/1.5 (!) charging scenario, “solar is not instant max output.”6)

Three Stage Charging

Three stage7) or “smart” chargers (whether solar charge controllers or converters) will follow a common pattern. Battery manufacturers publish specs for charging and a good charger will let the user configure the charging stages in accordance with that information. Less-expensive chargers may have presets for charging different battery types; if you get lucky one of the presets will match your manufacturer's charging recommendations.

SternWake sums up smart charging:

“Bulk rate is maximum amps the charging source can supply until the absorption voltage is reached, at that point the amps required to hold the ABSV8) will taper. The longer the battery is held at ABSV, the more the amps required to hold ABSV will taper. At some point, either time, or the amps required to hold ABSV fall below a threshold and triggers float mode.”9)

This graphic10) shows how current and voltage change during the full charge:


Charging won't be as neat as the graphic due to varying solar harvest, varying loads, etc. In practice “constant current” means “as much as your solar can produce”. Generators and shore power charging will usually be able to hold a steady level of current.

The example below will use charging data for a 12v bank of Trojan T-105 FLA batteries at 77F.11)

Bulk stage

The bulk stage is a fast and furious rush to get maximum power12) returned to the battery bank. Since the stage by definition requires all the power the system can generate13), this is when controllers typically get the most benefit from MPPT features.

This stage begins when charging starts (as when the sun comes up) and ends when the battery climbs to the acceptance voltage setpoint, 14.8v in our T-105 example.

Note: this is the stage where it is most useful to charge the bank with a generator or alternator. After bulk charging is complete the current required begins dropping automatically and roughly linearly.

Mythbuster: bulk charging lead banks with higher current does not necessarily shorten overall charging time. Once minimum charge rates are met, fully charging takes roughly the same amount of time.

charging current

Charging current is stated as a fraction of C; 0.1C is 10A for a 100Ah battery.

  • Trojan14)
    • AGM 0.2C
    • Gel 0.13C
    • Solar Premium and Signature: 0.13C when charging time is unlimited. “If charging time is limited [like with solar charging], contact Trojan Technical Support for assistance”
  • Renogy
    • LiFePO4 0.2C15)
    • AGM 0.4C max16) unless labeled with 0.3C max
    • GEL 0.3C17)
  • Rolls18)
    • Flooded 0.1C
    • AGM 0.2C (min 0.1C, max 0.3C)
    • GEL 0.2C (min 0.1C, max 0.3C)
  • Lifeline19)
    • AGM up to 5C inrush, 0.2C minimum20)
  • Odyssey21)
    • AGM 0.4C minimum, no maximum
  • EastPenn/Deka - 0.3C max22) -

Absorption stage

The absorption stage, sometimes called acceptance23) or boost24), is a constant voltage stage during which the battery is brought to full charge. This stage requires a great deal of time but decreasing amounts of current.

Absorption begins when the battery reaches the absorption voltage (Vabs, 14.8v in our example) and ends when the battery tapers off current acceptance25) to something like C/100 to C/5026) (“tail current”, “End Absorb”, “endAmps”), and/or when a period of time has elapsed.

Example: if you have a 100Ah battery bank and the manufacturer states that endAmps is C/100, then the battery is fully charged when it is only accepting 1A of current27) at Vabs. An endAmps recommendation of C/200 would be the battery accepting 0.5A of current.28) at Vabs.

In practice absorption takes longer when the battery has been discharged deeply and shorter when it has not.29)

Mythbuster: although it is commonly said that Vabs is attained at ~80% capacity…

charge current affects the SOC transition point from bulk to absorption charging - MaineSail30)

MaineSail found that

  • charging AGM at C/2.5 (max rate) resulted in the battery being 63.3% “full” at Vabs and being fully charged in 5hrs, 30 minutes
  • charging AGM at C/5 (min rate) resulted in the battery being 77.4% “full” at Vabs and being fully charged in 5hrs, 42 minutes

Solar-only charging at lower current levels may indeed mean 80% of the amps are replaced at the end of Bulk.31)

Some controllers will allow the user to configure the time or ratio of capacity/current (C/n). Sternwake says:

If your charge controller only holds [absorption] voltage for an hour or two, that is likely not enough time. As long as [there is a load] and you cycle the battery daily, you could set float voltage to [absorption voltage] without worry. Only when you stop cycling the battery do you need to return float voltage to more regular 13.2v32) levels. Premature application of float voltage by automatic charging sources is a battery killer.33)

If Absorption cannot be completed in the max amount of time configurable in the controller (due to damaged/old batteries), charging at the minimum rate and/or at higher voltage may help compensate.

Float stage

The float stage is not a charging stage but rather a maintenance stage.34) In it the battery is fed just enough current to hold the battery at a full charge. The battery will remain fully charged indefinitely in Float; it does not overcharge.

Common Vfloat values range between 13.2v for stored batteries to 13.8v for banks that are deep cycled each day.

also see Setting Vfloat to Vabs


Charging can be applied35) with a simpler methods like charge-and-hold where the charger has a single setpoint which it holds; current will drop as the battery accepts less.

Examples include:

  • RV converters, usually preset with a float-type setpoint like 13.4v. The slow charging associated with this voltage level is offset by the assumption that one is plugged into shore power.
  • power supplies, which will usually have a potentiometer (pot) to adjust voltage by +/- 10%. Shore power assumed.
  • alternator charging, where the alternator determines output voltage


Shunt (on/off) chargers have a charge-and-stop approach where they:

  1. charge to High Voltage Disconnect (Vhvd) and stop
  2. fall in voltage to a High Voltage Reconnect (Vhvr)
  3. repeat

This differs from PWM because PWM can hold an average voltage by cutting power hundreds or thousands of times a second. If you make a setpoint for 13.6v it will hold 13.6v as long as sun and loads cooperate. A shunt programmed to charge to disconnect at 13.6v and reconnect at 12.7v will average 13.05v.

These setpoints may or may not be user-configurable.

Charge-and-stop may be useful when charging battery chemistries like lithium, or when shallow-cycling.

Charging AGM batteries

AGM batteries are kept healthy by:

  1. vigorous Bulk charging36) sometimes greater than solar-only charging can provide.37) Consider augmenting with alternator or shore power charging.
  2. long Absorption stages38)
  3. typically lower Absorption voltage (to prevent outgassing)39)
  4. no Equalizing (which could create heavy outgassing)

Many charge controllers have AGM or GEL modes that handle these setpoints and durations.

Charging other chemistries

Lithium packs don't need Absorption or Float stages in the lead-battery sense but those voltage setpoints can be put to good use. Follow your Li battery mfg's advice on charging.

  1. Absorption voltage - 4S LFP packs are actually 100% state of charge at 13.8v, but Li manufacturers typically specify “absorption” charging a bit higher like 14.2v. This allows for quicker charging and also gives some overvoltage the BMS can use to “top balance” cells.40)
  2. Float voltage - Li does not like to sit at 100% SoC,41) so a float voltage in the 13s allows them to come down off the peak and rest at a healthier voltage for them while still holding useful capacity.

Also see LiFePO4 (lithium) bare cell charging and thoughts about charging LFP gently.


Sometimes called the fourth stage, “equalizing is an overcharge to stir up electrolyte in stationary deep-cycle flooded/wet batteries.42) Since most vandwellers are mobile and electrolyte gets agitated by the van's movement EQ may not be required for vandwellers. Information provided below for completeness.

An equalizing charge prevents battery stratification and reduces sulfation which are leading causes of battery failure. Trojan recommends equalizing every 30 days or when batteries have a low specific gravity reading after fully charging… Deep-cycle AGM or gel batteries should NEVER be equalized.”43)

House banks in rigs that are driven regularly are likely already mixed by jostling, so equalization may be less important.

Our theoretical T-105 bank will equalize at 16.2v. Since voltages are so high it is common to disconnect everything else from the battery during equalization; this prevents overvoltage damage to electronics. Overvolting electronics could be particularly expensive if the solar controller goes into equalization when alternator charging is occuring – the house and chassis sytems are combined and overvoltage could be sent back to the vehicle.

If water levels are below the plates, add enough water to cover the plates before equalizing. Then top off after equalization. Topping off before equalization could result in spillover and loss of electrolyte.

Temperature compensation

Accurate charging requires the charger know the temperature of the batteries being charged. In hot temps a full charge will require somewhat lower voltage. Cold temps will require higher voltage44), enough to trip overvoltage protection in some 12v gear.

Trojan gives the adjustment value as:

5.0 mV per cell / °C or 2.8 mV per cell / °F 45)

The charger makes temperature-based voltage adjustments based on one of three methods. From most accurate to least accurate:

  1. remote temperature probe attached to battery. Some choose to mount on the terminal and some mount to the side of the battery.
  2. ambient temperature as measured by the charger. This can be different from the actual battery temp due to position, relative height to the battery, or if the charger gets confused by its own heat.
  3. in the absence of actual measurements the charger will likely use the 77F laboratory standard.

This automatic voltage tweaking for temperature may result in your actual battery voltages being observably higher in cold temps and lower in hot temps.

voltage sensing and compensation

Relatively high current on a wire will cause the voltage to read high (while charging) or low (while discharging). This can affect proper charging, load handling, etc. Properly sensing and accounting for voltage deviation can make a major difference in the quality of a charging routine.

See this article for a list of known chargers with voltage sensing abilities.

Watering batteries

www.trojanbattery.com_wp-content_uploads_2013_07_waterdia_b.jpg Flooded lead-acid batteries outgas during Absorption, causing a slow loss of water.46) They outgas intentionally in Equalization. To counteract this, the 'dweller must remove the caps and inspect the water level. If low, distilled water is added.

  1. check to see if plates are covered with water. If not, add enough to cover the plates
  2. fully charge the battery
  3. add water, if needed, to bring level up to just below the “fill well” (where you add the water)

images-na.ssl-images-amazon.com_images_i_41ja5xov_2byl._sx90_.jpgTrojan has a YT video about battery maintenance, including watering.

A battery waterer can make the job easier, and does not require one to judge water level visually. The water “bloops” as it fills and stops blooping when the water is at the correct level.

Batteries that are older, in poorer health, or are charged at higher voltages will be “thirstier”. Check water levels 1x/month until you know how they behave.

Charging while using the 12v system

Smart chargers handle charging under load gracefully. As long as there is enough solar power coming in to hold the absorption or float voltage steady those charging stages will not be disturbed. If there is not enough power to hold the prescribed voltage the charger may restart Bulk charging. This behavior may be configurable by the user.

Example: a battery bank is in Float mode, 13.5v with minimal current, let's say 0.1A. A laptop charger is plugged in which pulls 3A. The controller will attempt to provide 13.5v at 3.1A. If it can do so the charger stays in Float mode. If it cannot hold that level it will drop back to Bulk stage.47) Morningstar explains it like this:

“Once in Float stage, loads can continue to draw power from the battery. In the event that the system load(s) exceed the solar charge current, the controller will no longer be able to maintain the battery at the Float set-point. Should the battery voltage remain below the Float set-point for a cumulative 60 minute period, the controller will exit Float stage and return to Bulk charging.”48)

Mains automotive battery chargers, even smart ones, can get confused by load during charging.49)

Manual chargers will not be affected by load as they are controlled by the user.

Battery voltage while charging

Battery Voltage = Battery Voc50) + (Charge Current x Ri51))

Battery manufacturer charging guidelines

If your battery does not come with charging instructions then you will have to figure out who actually made the battery, and follow their instructions:

Deep cycle battery manufacturers such as East Penn Manufacturing, EnerSys, Exide Technologies, GS Yuasa, and Johnson Controls dominate the global market52)

Notice how your brand of battery is not listed? Batteries may be made to spec for the companies who label and sell them, or just standard batteries rebadged with sellers' branding.

Collected recommendations

The following data has been collected as an overview. Read the linked documents and make your own decisions rather than relying on the collection.

Note: the voltages below are what you plug into the controller, and the controller is assumed to have temperature compensation to account for battery temperature.53)

Manufacturer Flooded AGM Gel Lithium
Crown54) Min: C/10
Max: C/555)
Vabs 14.5v
Vfloat 13.5v
Veq 15.5v
Mastervolt Min C/6.6656) - C/4
Max C/3.3357)
Vabs 14.25
Vfloat 13.8v
Min C/6.6658) - C/4
Max C/259)
Vabs 14.25v
Vfloat 13.8v
Min: n/a
Max C/3.3360)
Vabs 14.25v
Vfloat 13.5v
Rolls / Surrette Min C/10
Recommended C/5
Max C/3.3361)
Vabs 14.7v
Vfloat 13.8v
Temp comp 4mV/cell
Min C/10
Recommended C/5
Max C/3.33
Vabs 14.28v
Vfloat 13.28v
Temp comp 3mV/cell
Trojan Min C/10
Max C/7.762)
Vabs 14.82v
Vfloat 13.5v
Vequal 16.2v
Temp comp 5mV/cell
Min n/a
Max C/5
Vabs 14.4v
Vfloat 13.5v
Vequal n/a
Min n/a
Max C/7.763)
Vabs 14.4v
Vfloat 13.5v
Vequal n/a
US Battery64) Charge rate C/10
Vabs 14.7v
Vfloat 13.0v
Vequal 15.3v
Charge rate C/10
Vabs 14.7v
Vfloat 13.4v
Veq (same as Vabs)
Fullriver65) n/a Charge rate C/5
Vabs 14.7v
Vfloat 13.65v
East Penn / Deka66),67)
including GC10 (aka Duracell GC2) and GC15 (Duracell EGC2)
Max C/3.368)
Vabs 14.4V - 14.7V
Vfloat 13.8V - 14.1V
Veq 15.0v - 15.3v
Odyssey (by EnerSys)69),70) Min ~C/371)
Vabs 14.7v
Vfloat 13.6v
Energizer GC2 (by Johnson Controls)72) Vabs 14.8v
Vfloat 13.2v
Veq 15.6v
Interstate GC273) Min C/10
Max C/7.774)
Vabs 14.8v
Vfloat 13.5v
Veq 16.2v

further reading

some mfg claim 4 or more charging stages, including equalization or proprietary stages. These three are the essentials.
Vabs, absorption voltage
from an unknown Amazon seller
or max recommended power
up to the battery bank's ability to accept it
which makes sense
doesn't make sense!
that the battery is taking from the charging source
1A-2A per 100AH of capacity
100Ah/100 = 1A
100Ah/200 = 0.5A
An extreme version of this would be older converters that charge only at Vfloat – all the amps could be replaced given enough time.
for example - secessus
insert holy war here
with variable amounts of success
minimum current of C/5 up to C/3
due to speedier Bulk charging
there are exceptions
lower-voltage cells are given a chance to rise to their proper individual voltage of 3.45v
the cells degrade faster at 100%+ SoC
the electrolyte may stratify in stationary batteries, resulting in zones of stronger and weaker acid
AGM and other VRLA outgas to, but use recombination to remake water from the gases
it cannot “drop back” to Absorption since Vabs is by definition higher than Vfloat
resting voltage
internal resistance
controllers without external temp sensors are measuring controller temp
56) , 58)
57) , 61)
much higher than usual
30%, for “maximum lifespan”, somewhat lower than usual
62) , 63)
actually 40% of C/10 rate
electrical/12v/charging.txt · Last modified: 2023/09/16 12:22 by frater_secessus