Lead-acid battery charging: the theory

Writing about battery charging would be easy if there was one recipe which was independent of the conditions of use and valid for all types of lead-acid batteries. But this is not the case.

Additional complicating factors are that there is often more than one charging device connected to the battery - an alternator and a solar panel, for example - and that the net-charging current is unknown if the battery is being used to run electronic devices at the same time.

Voltage-limited charging is the best way to eliminate the influence of power-consumers as far as possible. And working with two voltage limits - the Absorption and Float voltage limits discussed later in this chapter - is the generally accepted method for recharging deeply discharged batteries as fast as possible.

Three step (I U° U) charging

The bulk charge

When starting to charge a battery, voltage immediately jumps to approx. 2.1 V / cell (12.6 V for a 12 V battery and 25.2 V for a 24 V battery) and then slowly rises until the first voltage limit is reached. This is the current-limited or ‘bulk’ phase of the charge cycle, during which the battery will accept all the available charge current.

For big battery banks it is advisable to limit the charge current to C / 5 - or even better - C / 10, by which we mean between one-fifth and one-tenth of the battery’s rated capacity. So a 100Ah battery should be charged at a rate of between 10 and 20 amps.

A deeply-discharged battery will accept a current of this order of magnitude until it is about 80 %

charged. It will then reach the first voltage limit. From there onwards, instead of “absorbing” all of the current being “offered”, charge acceptance reduces rapidly. Therefore this first voltage limit is called the absorption voltage and the subsequent phase of the charge cycle the absorption phase.

A high bulk-charging rate will heat the battery, increase gassing, and it will increase the absorption time needed to fully charge the battery. In other words: a high charging-current will only shorten the charging time to a limited extent.

In any case the charge current must be limited to C / 5 or less once the gassing voltage has been reached (at 20°C the gassing voltage is approximately 2.4 V / cell, or respectively 14.4 V and 28.8 V). Failure to reduce the charge rate at this stage will cause the active mass to be pushed out of the plates due to excessive gassing.

The absorption charge

When the pre-set absorption voltage limit has been reached, charging is limited to the amount of current that the battery will absorb at this voltage. During the absorption phase the current will steadily decrease as the battery reaches its fully-charged state in order to allow the diffusion process to take place.

The diffusion process explains a lot about charging and discharging batteries:

• When a battery has been subjected to a fast but shallow discharge, little diffusion deep inside
• the active material has taken place and the chemical reaction is limited to the surface of the plates. To recharge, a short - or even zero absorption period will be needed. (The battery in a car is charged at a fixed rate of 14 V). To recover from a long and deep discharge, a long absorption period will be needed in order to reconvert the active material deep inside the plates.
• Thin-plate starter-batteries need less absorption charging than thick plate or tubular plate heavy-duty batteries.
• Absorption is a trade off between voltage (increasing the voltage results in stronger electric fields which will increase diffusion speed) and time. Applying a high voltage will however heat up the battery, increase gassing to a level where the active material is pushed out of the plates and, in case of ‘maintenance free’ Valve Regulated Lead-acid batteries (VRLA) cause venting which will dry out and destroy the battery.

So what does this mean in terms of absorption voltage and absorption time? We can distinguish between 3 groups of batteries:

1. Flooded lead-antimony batteries: Here we have a rather wide trade-off band of absorption voltage against time, ranging from 2.33 V / cell (14 V) and a long absorption time to 2.6 V / cell (15.6 V) and a much shorter absorption time. To avoid excessive gassing, charge current should be limited to at most C / 5 (20 % of the rated capacity) or, even better, C / 10 of the capacity of the battery (for example 40 A for a 400 Ah battery) once the gassing voltage has been reached. This can be achieved by either current-limiting, or by limiting the rate-of-voltage-increase to about 0.1 V per cell per hour (0.6 V per hour for a 12 V battery or 1.2 V per hour for a 24 V battery). It is also important to know that batteries do not need to be fully recharged after every discharge. It is perfectly acceptable to recharge to 80 % or 90 % (partial state of charge operation, preferably including some gassing to limit stratification) on average, and to fully recharge once every month.
2. The Spiral cell Absorbed Glass Mat (AGM) battery stands apart because it is sealed and accepts a wide absorption voltage range.
3. Other VLRA batteries have a limited absorption voltage range that should never be exceeded. Higher voltages will result in venting. The battery will dry out and be destroyed.

The float charge

After the battery has been fully charged it is kept at a lower constant voltage to compensate for self-discharge, i. e. to keep it fully charged.

As mentioned earlier, if maintained for long periods of time (several months) the float voltage may not deviate more than 1 % from the voltage recommended by the manufacturer, after compensating for temperature.

Excessive voltage results in accelerated aging due to corrosion of the positive plates. The rate of positive plate grid corrosion will roughly double with every 50 mV of increase in cell voltage (0.3 V respectively 0.6 V for 12 V and 24 V batteries).

Insufficient voltage will not keep the battery fully charged - which will eventually cause sulphation.

Regarding float voltage, we must distinguish between flooded and VLRA batteries:

1. The recommendations for float-charging flooded batteries vary from 2.15 V to 2.33 V per cell

(12.9 V to 14 V for a 12 V battery). The flooded battery types that have been discussed have not been designed for float charging during storage for several months or years.

When float-charged at the higher end of the 2.15 V to 2.33 V range, service life will be shortened due to corrosion of the positive plate grids, and batteries with a high antimony content will need frequent topping up with de-mineralised water.

When float-charged at 2.15 V per cell, aging and gassing will be under control, but a regular refreshing charge at a higher (absorption) voltage will be needed to maintain the fully-charged state. In other words: the high end of the 2.15 V to 2.33 V range is fine for a few days or weeks, but not for a six-month winter period.

The following table shows how much water is lost due to gassing in case of a relatively new low antimony battery (gassing increases with age):

Gas generation and water consumption is based on a 6 cell (= 12 V) battery. The topping up interval is based upon 0.5 l of water lost per 100 Ah. The water surplus in the battery is approximately 1 l / 100 Ah.

The formulas:

• 1g of water can be decomposed into 1.85 l of oxygen + hydrogen gas
• 1 Ah “lost” due to gassing generates 3.7 l of gas in a 6 cell (= 12 V) battery

The table shows that a float voltage of 13.5 V (13.5 V is the frequently recommended float level for the flooded batteries under consideration here, as lower float voltages do not completely compensate for self-discharge) or higher will result in topping-up being required more than once a year. Please also note that batteries with more antimony doping will consume 2 to 5 times more water!

Instead of trying to find a delicate balance between insufficient voltage to compensate for self-discharge and too much gassing at a higher voltage, it would be better to leave the battery open-circuited (disconnected to any device) and to recharge, depending on temperature, at least once every 4 months; or to reduce float voltage to a very low level, for example 2.17 V per cell (13 V respectively 26 V), and also recharge regularly at a higher voltage.

2. All VLRA batteries mentioned can be float-charged for long periods of time - although some studies have shown that a treatment similar to the one proposed here for flooded batteries will increase service life.

Equalising

When not charged sufficiently, batteries will deteriorate for the following reasons:

• sulphation
• stratification - whereby the electrolyte is more concentrated at the bottom than at the top, leading to reduced capacity (flooded batteries only)
• cell imbalance

Batteries will in general reach their fully-charged state, including equalisation, during the absorption charge or when float-charged for a sufficiently long period of time.

If they have been used in partial state of discharge for some time, they will recover by:

• repetitive cycling and charging with the appropriate absorption voltage and time
• an absorption or float charge during a longer period of time
• a real equalisation charge (see below)

An equalising charge is performed by first charging the battery as usual, and then continuing the charging process with a low current (3 % to 5 % of its Ah capacity, i. e. 3 to 5 A for a 100 Ah battery) and allowing the voltage to increase to 15-16 V (30-32 V for a 24 V battery) until the specific gravity (SG) stops increasing. This will take 3 to 6 hours, and by then all cells should give the same reading. Be sure to isolate the battery from all loads sensitive to over-voltage during this period.Heavy-duty traction batteries, especially, will need periodic equalisation charges.

How often should a battery be equalised?

It all depends on type and usage. For batteries with high antimony doping, the best way to find out is to check Specific Gravity (SG) after a normal charge:

• If all cells are equal and at 1.28, there is no need to equalize
• If all cells are between 1.24 and 1.28, it would be good to equalize when convenient, but there is no urgency
• If the SG of some cells is less than 1.24, an equalization is recommended.
• If all cells are below 1.24, the battery is undercharged and the absorption time or voltage should be increased.

On VLRA batteries and low antimony flooded batteries the SG cannot be measured because the battery is either sealed, or because the reading will be unreliable. The easiest way to check if they are really charged to the full 100 % is to monitor the charge current during the absorption charge. The charge current should steadily decrease and then stabilise: a sign that the chemical transformation of the active mass has been completed and that the main remaining chemical activity is gassing (decomposition of water into oxygen and hydrogen).

Temperature compensation

Battery temperature is important when charging batteries. The gassing voltage and consequently the optimum absorption and float voltages are inversely proportional to temperature.

This means that in the case of a fixed charging voltage, a cold battery will be insufficiently charged and a hot battery will be overcharged.

Both effects are very harmful. Deviations of more than 1 % from the correct (temperature dependent) float voltage can result in a considerable reduction of service life (according to some studies up to 30 % when the battery is float charged for long periods of time), particularly if the voltage is too low and the battery does not reach or stay at 100 % charge, so that the plates start to sulphate.

On the other hand, over-voltage can lead to overheating, and an overheated battery can suffer “thermal runaway”. Because the gassing voltage decreases with increasing temperature, the absorption and float charge current will increase when the battery heats up, causing the battery to become even hotter. Thermal runaway quickly results in destruction of the battery (the excessive gassing pushes the active mass out of the plates), and there can be a risk of explosion due to internal short-circuits and high quantities of oxygen and hydrogen gas being emitted from the battery.

The charging voltage - as quoted by European battery manufacturers - applies at 20°C battery temperature and may be kept constant as long as the temperature of the battery remains reasonably constant (15°C to 25°C).

Although manufacturers’ recommendations differ to some extent, a temperature compensation of - 4 mV / °C per cell is a generally accepted average. This means – 24 mV / °C for a 12 V battery and – 48 mV / °C for a 24 V battery.

Where the manufacturer specifies an absorption voltage of, for example, 28.2 V at 20°C, then at 30°C the absorption voltage must be reduced to 27.7 V. This is a difference of 0.5 V that certainly cannot be neglected. When, in addition to an ambient temperature of 30°C, the internal temperature of the battery rises by a further 10°C - which is quite normal during charging - the absorption voltage must be reduced to 27.2 V. Without temperature compensation the charge voltage would have been 28.2 V which could quickly destroy a gel or AGM battery bank worth thousands of dollars!

What the above means is that temperature compensation is important, and must be implemented, especially on large, expensive house batteries, and when a high-rate of charge current is used.

All charging voltages mentioned here are subject to temperature compensation.

Overview

The following table gives an overview of how batteries can be recharged after a 50 % discharge. In practice recommendations can vary from one manufacturer to another and also depend on how the battery is used. Always ask your supplier for instructions!

Notes:

1. In practice, when shore power is not available, batteries on a boat, in a camper, or in an off-grid application, tend to be charged as fast as possible, with a shortened absorption time, or no absorption period at all (partial state of discharge operation). This is quite acceptable, as long as a charge to the full 100 % is applied regularly.
2. When charging at a voltage exceeding the gassing voltage, either the current must be limited to, at most, 5 % of the Ah capacity of the battery, or the charge process should be carefully monitored and the voltage reduced if the current tends to increase to more than 5 % of the Ah capacity.
3. When float-charging batteries at 2,17 V per cell a regular refreshing (Storage) charge will be needed.
4. With regard to service life and overcharging: Starter batteries, or bow thruster batteries are often charged in parallel with the house battery. The consequence is that these batteries will frequently be charged at a high voltage (15 V or even more) - even though they are already fully charged. If this is the case, VRLA batteries should not be used for this purpose because they will start venting and dry out. The exception is the spiral-cell VLRA battery, that can be charged at up to 15 V without venting.

Flooded and spiral cell batteries will survive - but age faster. The main ageing factor will be corrosion of the positive plate grid - the rate of which doubles for every 50 mV of voltage increase per cell. This means that an OPTIMA battery, for example - which would last 10 years at its recommended float voltage of 13.8 V - would age 4 times faster at 15 V (((15 – 13.8) / 6) / 0.05 = 4) …reducing service live to 2.5 years if constantly charged at 15 V. Similar results are obtained for flooded batteries. While this calculation is theory and has not been tested in practice it nevertheless shows that regular overcharging during short periods (in practice only during the absorption charge period of the house battery) of starter or bow thruster batteries does not decrease service life to an unacceptably low period.

Conclusion

There is no simple recipe that can be applied to all batteries and operating conditions. And it’s also worth noting that there is no greater variety of operating conditions, or battery types, than is found on a yacht.

Note: Because the topic is a complicated one, our own range of battery chargers have features and automations for the parameters we have discussed here - including optional or built-in temperature compensation; voltage sensing; multi-step charge algorithm; storage mode (follows 24 hours at Float charge) with weekly one hour boost; equalisation program …among others.