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A battery monitor should therefore compensate capacity for the rate of discharge.
In practice this is quite complicated because the discharge rate of a house battery will vary over time.
3.6. Is capacity “lost” at high rates of discharge?
3 cites the example of a battery where the rated capacity under a 20-hour discharge was 200 Ah,
thus C20 = 200 Ah. The corresponding discharge current is:
Under a discharge current of 200 A the battery was flat in 30 minutes. So although we started with a 200 Ah battery, it was flat after discharging only 100 Ah.
This does not mean that, with a discharge current of 200 A, the 100 Ah capacity difference (C20 - C1 = 200 – 100 = 100 Ah) has “disappeared”. What happens is that the chemical process (diffusion, see sect. 2.2.3.) is progressing too slowly, so that the voltage becomes unacceptably low. A battery discharged with 200 A and “flat” in 30 minutes will therefore also be (nearly) fully charged again after recharging 100 Ah, while the same battery which is discharged with I20 = 10 A and is flat in 20 hours will be nearly fully charged after recharging 200 Ah.
In fact a battery which has been discharged at a very high rate will recover over time and the remaining capacity can be retrieved after the battery has been left at rest for several hours or a day.
In my opinion, apart from a voltmeter and an alarm function, very useful features are event counting and data logging 3.7.1. Event counting
3.7.2. Data logging Data logging would mean that, in addition to specific events, at regular intervals the status of the battery is stored in order to be able to reproduce a history of use at a later date.
4.1. Introduction Writing about battery charging would be easy if there was one recipe, 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, and that the net charging current is not known because of consumers that are also connected to the battery.
Voltage limited charging is the best way to eliminate the influence of consumers as far as possible. And working with 2 voltage limits, the absorption and float voltage limits discussed later in this chapter, is a good and generally accepted method to charge batteries which have been deeply discharged, as fast as possible.
A further refinement of the standard 3 stage (bulk – absorption – float) method is adaptive charging: see sect 5.3.2.
4.2. Three step (I U° U) charging
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.
- 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 VRLA batteries, 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). See section 5.3.2.
It is also important to know that batteries do not need to be fully recharged after every discharge. It is very 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 AGM battery stands apart because it is sealed and nevertheless 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.
4.2.3. The float charge After the battery has been fully charged it is kept at a lower constant voltage to compensate for selfdischarge, 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 over long periods of time (i. e. several months or years).
When float charged at the higher end of the 2.15 V to 2.33 V range, service live will be shortened due to corrosion of the positive plate grids, and batteries with a high antimony content will need frequent topping up with demineralised 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 6 months winter period.
The table shows that a float voltage of 13.5 V (13.5 V is an often recommended float level for the flooded batteries under consideration here, as lower float voltages do not completely compensate selfdischarge) or higher will result in topping up needed more than once a year. Please also note that batteries with more antimony doping will consume 2 to 5 times more water!
To my opinion, instead of trying to find a delicate balance between insufficient voltage to compensate for self-discharge and to much gassing at a higher voltage, it would be better to leave the battery open circuited and 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. This regular refreshing charge should be a feature of the battery charger.
See section 5.3.2.
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 (see for example “Batterie Technik” by Heinz Wenzl, Expert Verlag, 1999).
On VLRA batteries and low antimony flooded batteries the SG cannot be measured, respectively 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).
4.4. Temperature compensation As has already been mentioned in sect. 2.5.9, temperature is of importance when charging batteries. The gassing voltage and consequently the optimum absorption and float voltages are inversely proportional to temperature.
This means that in 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 % of 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, and the battery becomes even hotter, etc. 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 coming out of the battery.
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 in this and in other chapters are subject to temperature compensation.
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!
1) In practice, when shore power is not available, batteries on a boat tend to be charged as fast as possible, with 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 (see sect. 4.3).
2) When charging at a voltage exceeding the gassing voltage, either the current should 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 charge will be needed.
4) About service life and overcharging:
Starter- or bow thruster batteries are often charged in parallel with the house battery (see sect. 5.2). The consequence is that these batteries will frequently be charged at a high voltage (15 V or even more) although 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 aging factor will be corrosion of the positive plate grid, and the corrosion rate 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 it would constantly be 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 live to an unacceptably low period.
As mentioned earlier, there is no simple recipe that can be applied to all batteries and operating conditions.
Also, there is no greater variety of operating conditions and types of batteries than can be found on a yacht.
To get a better idea of how batteries are used and what this means for charging, let us again take the example from section 2.4. Let us assume that the yacht has 3 batteries on board: a house battery, a starter battery and a bow thruster battery.
How are these different batteries used, and how should they be charged?
4.6.1. The house battery
1) Cyclic use, in the partial state of charge mode, when sailing or at anchor. Important here is charging as fast as the battery permits. Temperature compensation is a must to prevent early failure due to overheating and excessive gassing.
2) A mixture between float use and short, shallow discharges when motoring or moored. The risk here is that a 3-step alternator regulator (when motoring) or a charger, (when connected to shore power) is frequently triggered by these shallow discharges to go into bulk and then absorption mode. The result could be that the battery is continually subjected to absorption charging and will be overcharged.
Therefore, ideally, the length of the absorption phase should be in accordance with the preceding DoD.
See section 5.3.2. for the adaptive charging method, a Victron Energy innovation.
Flooded batteries, if being float charged without any discharge occurring, should be switched to the lower 2.17 V per cell level and be regularly topped up with an absorption charge at 2.4 V / cell or more.
Again, see section 5.3.2.
In practice however the starter battery will very often be charged in parallel with the house battery, which is acceptable as long as the right type of battery is used and some decrease of service live is accepted (see note, sect. 4.5).
4.6.3. The bow thruster battery When used, discharge can be deep, and fast recharge will be required. In general the most practical solution is to charge the bow thruster battery in parallel with the house battery. Often spiral-cell batteries are used, because of their very high peak current capability. These same batteries will accept a wide recharge voltage range and are very tolerant to overcharging.
5.1. The alternator The main engine of a boat is normally fitted with a standard automotive alternator. Standard automotive alternators have a built-in regulator with temperature compensation. The temperature is measured in the regulator itself. This is a suitable arrangement for cars, where the battery temperature will be roughly the same as the temperature of the regulator.
This charging system works perfectly given the following conditions:
- the battery is a flat-plate automotive battery
- the battery is nearly always fully charged
- the temperature difference between the regulator on the alternator and the battery is limited
- the voltage drop along the cable between battery and alternator is negligible (i.e. less than 0.1 V, including switches, isolators, etc.).
Problems occur as soon as one of the above conditions is no longer fulfilled.
The following sections shortly discuss the practice of charging batteries with an alternator.