Dr Mike McDonagh enters the battery-charging universe to examine what happens when charging multiple strings of batteries connected in parallel. Can you get something for free in this universe or are there hidden dangers lurking around the corner of convenience?
Whilst operating batteries in different configurations is a tried and tested practice, it is interesting to reflect on some of the common mistakes and misconceptions that are built into some companies’ and individuals’ working practices. Going back to the start of my working life (I hesitate to call it a career), I once asked a battery formation supervisor why he chose to operate so many circuits in parallel. His response was: “Because for every circuit charged you get another one charged for free.”
It was not until I had to check a new UPS telecoms installation, using multiple series strings connected in parallel, that I understood his position (Fig 1 above). There were, in fact, six parallel strings connected to a single charger outlet. The reason for this was that the number of fairly small 12V batteries was limited by the charging voltage output of 100V. The maximum voltage was restricted for safety reasons; but the current output was 120 amps, giving a theoretical maximum of 20 amps per series string.
In effect, a fixed voltage would not restrict the number of strings you could connect in series; the[ limitation would be the magnitude of the current per string. With a float charge UPS application, you can easily accommodate a reduced current output. So, from a particular point of view, it was additional free charging space. In the case of increasing to a 10-series string output, the maximum current available per string would be 12 amps— more than enough for a float charge application with an occasional deep discharge. However, free charging space does not mean free-of-cost charging.
As explained above, the attraction of parallel charging is manifest. It can greatly increase the number of batteries charged from a single charger output. This saves on capital cost and potentially increases productivity. However, it should also carry a warning label: “It’s not that simple”. Here are some of the potential consequences of parallel charging:
Charging series strings in parallel
- Current is diverted to those strings with a lower total resistance
- Lower resistance strings will get more Ah during charge than those with a higher resistance
- Those strings in a higher state of charge will discharge into those with a lower state of charge. This will slow down charging of the higher state-of-charge (SoC) strings
- In a series string, the lower voltage batteries or cells will not be charged to the same degree as the higher voltage ones, due to either their IR or their SoC
- The imbalance of Ah through parallel strings, or the voltage imbalance within these strings, will result in some batteries or cells being overcharged and others being undercharged, simultaneously in the same charging circuit
- Depending on the voltage settings on the charger, batteries or strings with a higher IR could be gassing, whilst the lower IR batteries are still converting AM from charging without gassing, despite the current being higher
Charging of single batteries in parallel
- Because batteries are single cells in series, this situation is similar to the parallel charging of strings of batteries
- Individual cells in a battery will be at different voltages as they will not have identical impedances. This means that some cells will be overcharged and some undercharged in voltage limited situations
- In the case of battery differences, the resistance of the connections and the cables between the batteries will add to the battery resistance. Differences in these connecting resistances will affect the voltage of the battery due to the current varying as a result of the total impedance of that parallel connection
- This can result in different voltages per battery despite their being connected in parallel. This will lead to undercharging of some batteries and overcharging of others
The following discussion provides the theoretical background and the practical experience that support the above statements. Firstly, understanding the basic principle of parallel series connection is straightforward. The principle is derived from Ohm’s law.
For series connections
V = I x R
Current = I through each resistance and throughout the entire line
Total line resistance = R1 + R2 + R3
Total line voltage = Vt = V1 + V2 + V3 = IR1 + IR2 + IR3
For series connections, the voltage is cumulative but the current is the same through each resistance.
For a parallel connected circuit
Vt is constant across each string
it = i1 + i2 + i3
rt = 1/r1 + 1/r2 + 1/r3
Vt = it x rt
In this case, it is the voltage that is the same for each connection and the current that varies.
The picture becomes a little more blurred when we consider parallel charging several strings of series-connected batteries. To fully appreciate how this can be troublesome, it is best to look at both the parallel-connected single batteries and the parallel-connected multi-battery strings described earlier.
Case 1: Multi battery series strings connected in parallel
A very common parallel charging construction is the series-parallel configuration used in telecoms towers and UPS applications. The configuration consists of a number of series strings attached in parallel for both charging and discharging. Fig 2 shows the arrangement; in this example, it is three series strings attached to a common load and charger. The batteries are divided into groups. There are three lines (1-3) that show a cross-section of three parallel-connected batteries. Then there are three strings (A, B, C) in which three batteries are connected in series.
There are advantages to this arrangement:
- Open circuit battery failure in a single line does not stop the operation due to maintaining the line voltage
- The larger the number of strings the less effect it has on the other batteries in the parallel strings
- The lower voltage operation makes it safer than an equivalent power series-connected application
- It also saves on capital cost as the inverter and charger are also lower voltage and more efficient at these voltages
- Total resistance is reduced when using parallel connections
There are operational disadvantages, however, and in the case of open-circuit failure in a series string, the other two circuits would have to increase their output under a constant load. This would result in the two working strings being discharged with an increased discharge current. This will result in the battery bank being unable to complete its cycle duty and may damage the remaining batteries.
In addition, the voltage across each battery string A, B and C is the same, but within each series string, the batteries may be at different voltages, depending on their individual impedances. This can lead to under and overcharging of batteries in a series string; this would lead to batteries going out of balance with the possibility of permanent damage.
Case 2: Single batteries per string
It may appear on the surface that if we have just a single battery per string, rather than several, then the voltage across the battery will be the same in lines 1-3. Any differences in voltage will be compensated on connection by the higher voltage batteries discharging into the lower voltage ones, until there is a negligible EMF difference. This is partially true, and, providing the batteries are left long enough, there will be a levelling up of the battery voltages giving a theoretical equal starting point for the charging process. However, we do not operate in an ideal world and there are always differences in battery internal resistance. Generally, chargers are turned on immediately after the batteries are connected, which limits the time available for any self-levelling mechanism.
To minimise these problems, it is difficult to overestimate the importance of having batteries of the same internal resistance when charging in parallel. We also have to add in the circuit resistance of each string. Despite there only being one battery per string, there are cables and connectors attached to each battery (Fig 3). Each of these strings will have a total resistance made up of those components, i.e. the battery internal resistance R1,2,3, the connecting cables r1,3,5 and the connections themselves r2,4,6.
Taking parallel string A from Fig 4, we can add up the resistance contributions from each terminal connector and cable, see Fig 5.
Applying this nomenclature to all the strings we can derive the following relationships for the whole circuit:
RL1 = r1 + R1 + r2
RL2 = r3 + R2 + r4
RL3 = r5 + R3 + r6
The total current output from the charger, It, will be shared between the lines according to their respective resistances:
It = V/RL1 + V/RL2 + V/RL3
The voltage is the same across every line so the current through each line can be defined as:
I1 = V/(r1 + R1 + r2)
I2 = V/(r3 + R2 + r4)
I3 = V/(r5 + R3 + r6)
The implications for the charging voltage for each battery in each line can be summarised as:
Line 1: V = v1 + V1 + v2 Where V = total voltage, v1 = left connection and cable V1 = battery one and v2 = right battery connection and cable.
Line 2: V = v3 + V2 + v4 Where V = total voltage, v3 = left connection and cable V2 = battery two and v4 = right battery connection and cable.
Line 3: V = v5 + V3 + v6 Where V = total voltage, v5 = left connection and cable V3 = battery three and v6 = right battery connection and cable.
The voltage across the battery will depend not only on the two obvious parameters of current and internal resistance but also on the limiting value of the total voltage of the parallel line. When there is voltage limited charging, the voltage of the battery will be limited to the total available voltage, minus the sum of the voltages created by the other resistive parts of the circuit.
Taking line one in Fig 2 as an example, the voltage across the battery can be calculated as:
V = I1 x (r1 + R1 + r2) where the voltage across the battery is:
V1 = V – I1 x (r1+ r2)
There is an available voltage for the battery, dependent on the accumulated resistance of the connections and wires as well as the line current and internal battery resistance. The higher the line resistance the lower the current and the lower the battery voltage in that line.
The line current is affected by the total line resistance, including that of the battery. Because of this, many companies have recommended criteria for batteries chosen to be connected in series-parallel circuits. The battery parameters that can have an impact on its internal resistance or impedance (R1-3 in our equations) can be listed as:
-
State of charge (Fig 6)
- Battery age
- Battery cycle duty
- Production batch
- Previous use history
- Electrolyte SG
- The measured impedance of a new battery at 100% SoC
Many manufacturers and battery suppliers, if they are aware of their products being used in a parallel or parallel series configuration, will rank batteries according to the above criteria. They will supply groups of batteries that have similar backgrounds, to ensure minimum voltage and current variation across the batteries’ terminals, when connected in a series-parallel configuration. If, for example, a VRLA battery with a fully charged IR of 6m-ohms was paired with a similar battery at 20% SoC with an IR of 16m-ohms, there would be a voltage difference according to the applied current. If that were 20A in a parallel arrangement, then the on-charge voltage difference would be dV where dV = 0.01 x 20 = 0.2 mv. This is a significant difference when considering a voltage-dependent step, such as a gassing phase in the charging profile. Of course, the other consideration is the current difference that would occur in each parallel line due to the individual batteries’ IR. The lower the battery IR the higher the current, the more Ah that would be put into that battery, assuming all other components were of equal resistance.
The other consequence of parallel charging is the lower current available to charge each battery. Unless the charger used is specifically designed for multiple batteries in parallel, the output current will be far lower than that designed to recharge a battery in a reasonable time. For example, if a designated charger for a single battery is designed to provide a full recharge in 10 hours then it will need approximately 10% more Ah than the capacity of the battery. In the case of a 200Ah battery that would be a total of 220Ah in 10 hours. The average current then would be around 22A, with big variations between bulk phase and gassing phase charging. For two identical batteries being charged in parallel on the same charger, that would mean doubling the time to 20 hours, or 40 hours for four batteries and so on for multiple parallel connections.
In summary, despite the perceived advantage of getting extra ‘free’ space in a charging circuit, there are real dangers in adopting this measure as a routine practice. The fact that has to be appreciated is that it is not only long, multi-battery strings connected in parallel that present danger; it is also parallel strings of single batteries if the batteries and the connecting wiring are not virtually identical. Apart from the inconvenience of longer charging times, there is also the distinct danger of over- and under-charging batteries due to differences in SoC, battery internal resistance and the resistance of the connections and wiring in the circuits. Taking all factors into account, parallel-connected battery charging may often be convenient, but if not done correctly could be disastrous.