The electro-chemical cells consistsof two terminal suspended in an electrolyte. The terminals are called the anode (-) and the cathode (+). An electrical current is essentially a flow of electrons, and the battery can be regarded as an electron pump. The chemical reaction between the anode and the electrolyte forces electrons out of the electrolyte and into the anode metal, through the circuit, then back to the cathode. From the cathode metal, the electrons re-enter the electrolyte. This direction may seems strange, from negative to positive. We regard ‘current’ as flowing from positive down to negative, but in fact, this current is a flow of electrons in the opposite direction! The anode and cathode both get converted during this reaction, one is ‘eaten away’, and the other has a build-up of material on it. When a rechargeable battery is recharged, this chemical reaction is reversed, and the terminals are restored.
The energy stored in a battery is measured in the units Ampere-hours (Ah). The units of energy are actually Volts × Amps × Seconds, but since the voltage of the cell is constant, it is only the product of current and time which determines the amount of energy in the battery.
Rechargeable cells generally have a lower energy density (that is the total amount of energy they can hold, Volts × current × time, divided by the physical size of the cell) than primary cells. A nicad D cell provides 5Ah at 1.2V, a lead acid D cell provides 2.5Ah at 2V, and the comparable alkaline cell provides 10Ah at 1.5V.
To provide a reasonable energy for a heavy-weight robot wars bout requires a minimum of 5 Ah realistically, It would require many Nicad or NiMH cells to achieve this, so the rest of this page will concentrate on lead-acid batteries, which most people are using.
The sealed type of lead-acid battery (SLA battery) is similar to the non-sealed type except a few changes in the electrolyte. This commonly takes the form of a gel or is absorbed by the separator. Methods to recombine oxygen formed at overcharging are employed. Although specifically designed for the reduction of excess gassing, pressure valves usually are installed to vent excess pressures to the atmosphere. The sealed construction allows a maintenance-free life which is not restricted to a horizontal orientation.
All lead-acid batteries produce hydrogen and oxygen during the recharging process. This production is increased if overcharging occurs. Sealed designs cause recombination of the oxygen at the same rate is produced, therefore eliminating the explosive mixture. Hydrogen which is produced will permeate plastic containers and as long as the sealed battery is not in a sealed small area, the hydrogen will harmlessly dissipate into the atmosphere. It is always good practice to allow for ventilation even with sealed batteries because of the possibility of a charger failure causing abnormal charge rates. If this happens, the battery will vent to prevent pressure build up. Another precaution is to prevent short circuits of the battery terminals. This can cause high heat and potential fire hazard.
Have a look at the top line in the graph, this is for the 70 Ah battery in the Hawker Genesis range. At the C/10 current, 7 Amps, looking along the graph we see that the battery will last 10 hours at this current, that gives 7A × 10hr = 70Ah. At 70 Amps, we may expect the battery to last 1 hour, but if you look closely, it will only last for 0.8 hours (48 minutes), that’s 56Ah. At 280 Amps drawn (a current of 4C), it will only last about 0.11 hours (6.6 minutes), that’s about 31Ah.
Taking the 26Ah line and plotting the percentage of nominal capacity against the current drawn, we have the following graph:
The I20 value is the current I = C ÷ 20, which for a 26Ah battery is 1.3 Amps.
Thus when we come to work out how large a battery we need, we are going to have to work out an approximate load-profile during the course of a battle. This is done later in this page.
Not only does load current reduce the rating, the temperature does also. Batteries are normally quoted for capacity at 25ºC. Below this temperature, and also far above it, they will operate less efficiently, The graph below shows how temperature affects the Hawker Genesis range of lead-acid batteries.
Also, the effective capacity of the battery improves as the temperature increases:
However, this doesn’t mean "the hotter the better"! The grid-corrosion rate in a battery is intimately linked to the battery’s ambient temperature. The higher the temperature, the greater the corrosion rate—and the sooner the failure of the battery. This accelerated corrosion at higher temperatures occurs regardless of the charge current flowing into the battery. However, since higher temperatures give rise to increased currents at a given voltage setting, the net result of an elevated battery ambient temperature is to intensify the negative effects on the battery.
It can be seen that lead-acid batteries have a reasonably flat discharge curve until they are nearly exhausted, although they are nowhere near as good as the NiCd secondary, and silver, and mercury primary cells.
Since the voltage does gradually decrease as the battery is discharged, an approximate determination of the state of discharge van be made by measuring the open-circuit voltage to within a tolerance of about 15%:
Next in importance to DOD in determining the battery’s cycle life is cycle time. The time allowed for a recharge between discharges is critical to the battery’s life expectancy. Generally speaking, the longer the time allowed for a recharge, the longer the battery’s life expectancy. The preceding paragraph may be better understood if one remembers that a battery is fully recharged when between 105 to 110% of the ampere-hours discharged are put back in the recharge. If the time allowed for a recharge is less, then the current magnitude for a given ampere-hour must be increased. An increase in the charge current can be accomplished only if the charge voltage is also increased. This, in turn, leads to a higher level of overcharge, which speeds up the battery’s ageing process.
Thus, the longer the recharge time, the lower the overcharge rate, and the better it is for the battery. Conversely, the shorter the recharge time, the higher the overcharge and the harsher the condition for the battery.
Therefore, if the capacity loss associated with self-discharge is not replenished, the battery ultimately fails because storage is equivalent to a very low rate of discharge. The key factor influencing the self-discharge rate is the storage temperature. This is because the ambient temperature plays a major role in determining the speed at which the internal chemical reaction proceeds. The higher the temperature, the faster the speed of chemical reactions.
As the battery charges up, its terminal voltage increases, and the internal resistance decreases. Eventually, when it is fully charged, it will be taking a trickle current from the charge which maintains its fully charged state. The graph below shows a constant voltage charge cycle with no current limiting.
Note that the voltage is dipped at the beginning. This is because during a constant current charge, the charger will supply the correct voltage to satisfy the Ohm’s law equation V = IchRbatt. The internal resistance of an uncharged battery is higher than that of a charged battery, so as the battery starts to charge up, its internal resistance decreases, and so the voltage consequently increases.
Obviously, by limiting the initial current, this increases the amount of time it takes to charge the battery.
This method must only be used for batteries that are specified to use it, and only under careful control of the final constant current. If the final current is too high, the voltage will exceed the ‘gassing voltage’, which means the recharging chemical reaction produces gases faster than the battery can recombine them, and the pressure seal may blow, damaging the battery.
Since the electrolyte in a SLA battery is inaccessible, the ambient temperature around the battery must be used. The charge voltage is then adjusted according to a graph like the one below. The adjustment may be different for each battery type or manufacturer, so you must refer to the battery datasheet for details.
The constant voltage scheme is the simplest to implement, but you must make sure your charger can handle the high initial currents, or is suitably current limited. The IUI scheme requires more complicated control and may need a special charger chip or microprocessor.
The main problem with them is the amount of ripple current that they generate. First, there will be less ripple if a full-wave rectifier circuit is used rather than a half-wave rectifier. The circuits below show how the half-wave rectifier can be modified to full-wave, using three more diodes of the equivalent type to that used in the half-wave rectifier.
The second alteration we can make is to filter the signal. This requires as large a capacitor as possible tobe connected to the output of the charger, in parallel with the connected battery. The charger itself should limit the current that will be dumped into this capacitor on power-up. Some care may be needed here however, since car battery chargers often only have current limiting based on the resistance of the wires in the secondary of the transformer!
They also produce the BQ2031 which is a switch-mode charger control IC. This can charge using a dual-voltage scheme, a dual-current scheme, or the pulse scheme described above.
Both these ICs have temperature compensation capabilities built in, and various safety and overcharge mechanisms to prevent damage to the battery.
Batteries may be connected in series to generate higher voltages. The total voltage is the sum of all the individual voltages. However, some care must be taken depending on exactly what load is attached to each battery. Take the following connections:
In connection A, two batteries are connected in series to supply a single load. The batteries should be of the same type and capacity, and should be at the same state of charge,i.e. when you connect them up, make sure both are charged up.
In connection B, it doesn’t matter if the two batteries are of different type. The upper 12v battery may be just a small one to supply high voltages to turn MOSFETs in the upper arms of an H-bridge for example. The low one will be the high power battery.
If you connect batteries in parallel you must be very careful that they are exactly the same type, same state of charge, and the same age, much more so than in the serial connection above. Batteries have an internal resistance, which goes up as they discharge. Imagine the following connection where one battery is fully charged and the other is discharged:
The discharged battery on the right has a terminal voltage of 10.5V and an internal resistance of 8 milliohms. The fully charged battery on the left has a terminal voltage of 13.2V and an internal resistance of 4 milliohms. The internal resistances are shown in the circuit for clarity. When the two batteries are connected together in parallel, the fully charged one will start charging the discharged one. A current will flow around the loop between the two batteries of
This current is very high and may damage some batteries.
If the batteries are at an identical state of charge, their terminal voltages will be the same, and so no appreciable current will flow when they are connected in parallel. Any imparity will be evened up as the one with greater charge and hence terminal voltage charges up the other.
It may be easier in the long run if you want to use two batteries with two motors to connect them separately. The negative terminals can be connected together, but the positives should then go to the positive connector of two separate speed controllers.
Batteries can be connected up in series-parallel configurations to get the required voltage and current capability, although it is recommended that no more than 4 in series should then be connected to a second chain of four:
When batteries are connected in parallel, they will not share current exactly. If one supplies a slightly higher current, then it will get warmer. As it gets warmer, it will become more efficient, and it will start to supply even more of the current. Hence there is the possibility that it will end up supplying most of the current while the other sits there getting charged up! A solution to this is to insert small value resistors in series with each battery before paralleling them up.
Remember, however, that Robot Wars has a voltage limit of 36 Volts DC!
If you have batteries in series, the maximum current you can take is equal to the maximum current of the least capable battery in the series chain (although as stated elsewhere in this page, all your batteries in series should be the same type anyway). If you have batteries in parallel, you can’t just double the maximum current capability, as explained in the series and parallel connection section, they will not current share very well.
What we need to do is determine a typical load profile during the bout. This is going to need examination of several typical bouts, and timing of the periods spent stationary (but ignoring dead robots!), accelerating, pushing and shoving, and at constant speed. All these phases of the bout represent a different load on the battery. Some estimates are shown in the table. (NOTE: I would appreciate it if someone who has several videos of robot wars could give more accurate average figures for this table). The current values are based on using two 12 Volt, 750 Watt motors, which seems to be a fairly typical setup.
The graph below shows the effective derating of battery capacity for the Sonnenschein Dryfit A500 battery based on the load current.
Using this graph, we can work out how much capacity is taken out of the battery for each phase of the bout. The capacity used up is the actual ampere-hours, divided by the derating factor. For example, when accelerating, we assume a derating of 50%, and this goes on for 20 seconds. The actual ampere-hours taken in this time is 100 Amps x 0.0056 hours = 0.56Ah, but the battery is only operating at an efficiency of 50%, so this takes 0.56 ÷ 50% = 1.12 Ah off the nominal ampere-hour rating. The Capacity used up column is therefore given by the equation:
where the 3600 is to convert seconds into hours. Note that the percentage value is converted to a fractional value before being used in the equation!
Total capacity used
Therefore, for this robot, we require a battery of at least 16 ampere-hours nominal capacity.
What assumptions have we made in this calculation which affect the result?
So here's the big question. There has been much debate over the years on the Robot Wars forum about which battery is the best to use. It has been fairly universally agreed that buying second batteries is a bad idea. Many people have done this and have ended up with poorly performing batteries. SLAs are easy to damage, and it appears that many second hand batteries have been treated badly.
As to which new battery to buy, there are pros and cons for all of them. The various opinions that are based on experience that I have seen on the forum are summarised in the table below:
|Hawker||Genesis||Very high charging current allowed.
Good efficiency at high currents
|Kiel||Cheap||Low power to weight ratio
High internal resistance
|Sonnenschein||Dryfit||Not enough information||Not enough information|
Andrew Marchant from Team Tornado has performed battery discharge tests on a variety of the common SLAs used by roboteers. The results can be found here.
sci.electronics battery FAQ
Ibex make battery chargers and gave lots of useful information on their site.
24Ah SLA datasheet:
A Supplier of Hawker batteries:
Battery charging documents:
Improved Charging Methods for Lead-Acid Batteries Using the UC3906:
A 3 Amp SLA battery charger project, very well laid out and explained.
High efficiency battery charger using power components.
A commercial battery charger manufacturer
A commercial battery charger manufacturer. Not sure whether any of these are suitable for
SLA batteries though, even though they say they are suitable for all lead acid batteries.
Linear Technology battery charger page, and a SLA charger circuit.
SmartKit of Greece
A dual rate charger (amongst other things)
WICEN (Wireless Institute Civil Emergency Network) SLA battery charger
[C1] US supplier of many types of batteries. Limited technical information on the site.