Unless your robot is frugal enough to make do with primary cells without breaking the bank, or is environmentally-friendly and runs off solar panels, it will probably need to use rechargeable batteries as its energy source.
Although very many chargers are currently available, they’re not always suitable for our needs, in terms of the types and number of batteries they can handle. What’s more, certain of them do not take very good care of the batteries entrusted to them, which can seriously shorten their life.
So this article proposes building your own tailor-made charger, using an IC that’s already old, but still very much current: the MAX713 from Maxim. As all robots
are different, we’re not going to suggest a completely finished circuit, but will instead explain how to adapt it to suit the characteristics of the batteries you’ll be wanting to recharge.
The MAX713’s basic application circuit is shown in the figure, but as you can see, certain elements have no values shown. In addition, there are various configuration links. Via these various elements, the MAX713 lets you charge from one to 16 cells (a cell is a basic 1.2 V element), define the charging current, define the end-of-charge float current, and lastly, select the mode for detecting end of charge. As far as the latter is concerned, and so as to be compatible with any batteries you are likely to use in your robot, we’ve left out the temperature detection method, which requires a thermal sensor (NTC or equivalent) inside the battery. So resistors R4 and R5 in conjunction with
the hard-wired links to inputs THI and TLO program the MAX713 into the mode that detects end of charge by voltage variation.
So now let’s see how to determine the other elements that are still open to you, so you’ll be able to build a charger that’s just right for your needs. Note right away that the configuration links can either be hard-wired on the PCB that you’ll be designing for your charger, or else connected to multi-way switches to create a multi-purpose charger.
You first need to decide 16., the charging current for your batteries, whose capacity C is expressed in ampere-hours (Ah). This can be calculated from: /fast = C/t where t is the desired charging time in hours. Watch out! The MAX713 does not handle times over 4 hours. And take care not to pick a value for /fast above 4C, which is currently the maximum current permitted for
fast-charging NiCad and NiMH batteries. If you are able to choose a lower current, so much the better, it will prolong battery life. Program this charging time by wiring pins PGM2 and PGM3 of the MAX713 as per Table 1.
Then choose the number of cells to be charged at the same time. For block batteries, you can find the number of cells by dividing the nominal voltage of the battery by 1.2 V. So a 9.6 V battery will contain eight cells. If the number of cells is 11 or more, the circuit can’t be used as is, and in that case it’s better to charge your batteries in two goes. Program this number by wiring pins PGMO and PGM1 of the MAX713 as per Table 2.
Then choose the unstabilized DC supply voltage for your charger (VA in the figure) so that it is at least 1.5 V higher than the maximum voltage of the battery to be charged. If your battery has less than four cells, this rule no longer applies, as the MAX713 supply has to be a minimum of 6V.
Then determine the maximum power dissipated in T1 using the following equation:
PD = (VA— VBATTmird x /fast
where VBAnmin is the minimum voltage of the battery to be charged. Choose T1 accordingly, if necessary fitting it with an appropriate heatsink.
Then determine the value of resistor R1 so the current drawn by the MAX713 will be 5 to 20 mA, using the equation: R1 = (VA— 5)/ / where / is between 5 and 20 mA.
Lastly, determine the value of resistor R6 by using the equation: R6 = 0.25 / /fan and its power by using PR6 = 0.5/fast (theoretically 0.25/fast in fact, but it’s best to use
a safety factor of 2, hence the modified equation).
Your charger is now operational, and is extremely simple to use; but because of the processors inside the MAX713, it is essential to make the connections to PGMO to PGM3 before applying power to the circuit, otherwise they may not be taken into account correctly. This is no problem for a hard-wired circuit, but if your charger uses configuration switches at this point, you’ll need to power down and power up again to confirm any configuration changes
made via these switches.
The LED lights when the charger is in fast-charge mode (at the current /fast determined above). It goes out when fast-charging is over and the charger goes into float charge mode. The current generated in this mode is sufficiently low that the battery may be left connected to the charger indefinitely if necessary.
To make sure our explanation is crystal-clear, here by way of example are the calculations for a charger for a pack of four 1.2 V NiMH batteries with a capacity of
1,800 mAh that we want to recharge in two hours.
- Calculate /fast: /fast = C/t,e., 1.8/2 = 0.9 A or 900 mA.
- PGM2 and PGM3 connections:
PGM2 tied to BATT— and PGM3 tied to REF, as we want a charge time of 2 hours, i.e. 120 minutes (in fact, we’ll get a maximum of 132 minutes).
- PGMO and PGM1 connections:
PGMO to V+ and PGM1 to BATT— since our battery comprises four cells.
- Determine VA: VA =3 V minimum.We’ll choose 9 V, to obviate any problems with possible supply voltage variations.
- Power dissipated in Ti: PD = (9 — 4*) .9, i.e. 4.5 W. So we’ll choose, for example, a TIP32A, giving us an excellent safety margin (PD„.„ = 40 W).
* fully discharged battery voltage estimated at 4 V.
- Calculate R1: R1 = (9-5)0.01**, = 400 LI We’ll use the closest preferred value,e. 390 a
**: a current of 10 mA was chosen.
- Calculate R6: R6 =25/0.9, = 0.27 LI.
- Calculate the power in R6: PR6 = 0.5 x 0.9 = 0.45 W. So a 0.50-W type is going to be fine.
As you can see, it’s taken us all of five minutes to produce a charger tailored perfectly to our needs. Now it’s your turn…