An Auxiliary Battery Pack for the Hilti TE 10A Hammer Drill

By Pierre Millo
Translated from the French by Peter Penczer,

I: General Observations

- Feed voltage: 36v at high speed, 13.2v at low speed (the ground is +36v).

- Amperage: 9 to 10 amps up to 15 amps depending on the conditions of use.

- There is some electronic circuitry in the Hilti battery that automatically interrupts use when the batteries are about 85% discharged.

- The original charger is made in such a way that the temperature of the elements being charged can’t go over about 40C.

II. The Hilti battery

Note: The old model doesn’t have the 13.2v intermediate plug, its capacity of 1.2Ah was raised to 1.4Ah. The fundamental difference for cavers is that on the old model no voltage is available on the two contacts without making connections simulating the drill, whereas on the new models a 13.2v source is available. Don’t use it too much, because it discharges only some of the elements.

- Schematic of one way of using it (without taking anything apart)

For those who want to use the 36v battery for something else (a lantern for example).

Figure_Millo_1.gif (5752 bytes)

View of the contacts on a Hilti battery

Short circuit 1 and 3: one hears a relay close inside, which causes the appearance of 36v on contacts 1 and 2. The relay cuts off the current automatically before a total discharge, when the voltage of the batteries falls to 30v. One has available a 36v, 10 amp source protected in amperage, and with a capacity of 1.4 * 0.85 = 1.2 amp/hours.

Note: Don’t let 1 and 2 short-circuit for no reason, because the wiring inside consumes 150 mA. A charged battery would therefore discharge in 1.2/0.15=8 hours.

- Schematic of charging (without taking anything apart)

For those who want to charge a battery with something else than the original charger (for example, for adapting a slow charger that would lengthen the life of the batteries)

As one can see on the schematic below, it is sufficient to connect the charger to contacts 1 and 2 (positive on 1 and negative on 2), because there is a diode that allows direct charging but prevents discharge without passing through the relay inside the battery.

III: The Hilti charger

It stops automatically when the temperature of the batteries becomes excessive. It locks itself in standby mode (the LED is lit continuously) and maintains a very weak charging current.

- Schematic of one use (without taking anything apart)

For those who want to use the charger for another battery

Figure_Millo_2.gif (6489 bytes)

View of the contacts on a Hilti charger

[?] = translation uncertain

Insert a rod into the middle, to trigger the on/off switch. Current for charging is available on the two large contacts.

Note: If the batteries are similar to those mounted in the Hilti pack, you can put a 6.8W thermistor (at 20C) in the middle of them, and connect it between the small contact and the larger one; it will stop automatically when it is charged. But if the batteries are different, there temperature has nothing to do with the state of their charge with this charger, therefore using a thermistor is useless and the charger will give full power all the time.

Be careful not to pass the amperage listed by the manufacturer (0.7A). An overamperage is probable when charging batteries larger than the originals. One can limit the charger easily without taking it apart, simply by connecting a resistor in series with the battery to the charger (several ohms). The original battery is charged with peaks of 5.5A when it is flat. You’ll have to choose a resistor that gives the same value.

IV: The Drill

- Schematic of how it works

For those who want to run the drill with another source of electricity than the Hilti battery

The wiring is very simple and can be done on the outside on the contacts, or on the inside, and running the wires through the air holes in the base of the handle.

Figure_Millo_3.gif (5927 bytes)

View of the contacts on a Hilti TE 10A drill

We did some performance tests under laboratory conditions. The electric power consumed is on average 350W. It varies a lot with how hard you push, the dust in the hole, etc. The best results are obtained when you push lightly, as if the drill, equipped with its battery, was vertical. Understand that it is absolutely necessary to frequently remove the dust from the holes while drilling, by pulling out the bit while running the drill. For a caver who is serious about digging, it is instructive to spend a few hours outside, to put an ampmeter in series on one of the large contacts, and realize at what point a light binding in the hole, pushing too hard, or too much dust, can be catastrophic for battery life. Another useful thing to do is measure the time it takes to drill a hole. You can then make a catalog of performance based on the rock. Thus, knowing your battery life in minutes of use and the nature of the rock, you can predict how many holes you can make. You will also have an objective way of measuring wear on the bit and on the drill.

V: Description of the 7Ah NiCad battery

1) In general

The shorter life and the performance of the lead batteries has led us to package much better performing NiCad batteries for digging, but also a lot more expensive. A pack of 4.4Ah 36v NiCads shockproof, sealed, and electronically protected costs about 2500 francs [US$333 @ 7.5 f/$]. A 7Ah pack with analog conditioning costs about 3500 francs [US$466].

2) List of necessary materials (7Ah battery)

- 3 packs of 12v 7Ah NiCad batteries (made by SAFT, VARTA, Sanyo…)

- HILTI electronic circuitry either recycled or bought new, it can be found inside the original battery pack, measures about 3 cm on each side and costs less than 150 francs [US$20].

- 1 10 amp diode, preferably of the large metal type

- a flexible 3x2.5 wire [presumably three conductors, with 2.5 being a metric equivalent to gauge], 2 or 3 meters long

- 1 waterproof brass or plastic grommet for the 3x2.5 wire

- 1 dry bag

- 1 TSA brand drill bag

- 1 closed cell camping pad, 10mm thick

- Misc.: aluminum sheet, screws, wire

Figure_Millo_4.gif (20548 bytes)

3) Wiring schematic

Figure_Millo_5.gif (52139 bytes)

Battery pack construction

1 7Ah 12v NiCad battery pack

2 TSA brand drill bag

3 Closed-cell pad for camping, 10mm thick

4 Dry sack (only one hole is necessary, for passing the grommet through)

5 Aluminum sheet of 3 [presumably a metric equivalent of gauge] with two 90 bends and a hole for the grommet

6 Sealant (protection)

7 HILTI electronic circuitry held by screws

8 Diode fixed on the metal sheet (for cooling)

9 Watertight screw-in grommet

10 Empty plastic tube to store the wire

11 Electric wire (3x2.5)

Approximate weight is eight kilos (17.6 pounds).

- Make sure there aren’t any rough spots, in particular, it is necessary to countersink the drill holes so as to not damage the dry sack. Over the long run, a sharp angle can cut the dry sack.

- Bind the battery packs and the aluminum plate solidly together with adhesive tape

- The three wires that lead outside are the positive, the negative, and the triggering wire which, put in contact with the positive, releases the power. You can use a standard 220v plug as a connector with the drill.

5) Conclusions and performance

The ensemble is shock proof and submersible under several meters of water without a problem. Its relatively high cost is justified by its reliability, its longevity, and its long range. To evaluate its performance, you can look at what’s written in paragraph IV.

Reasonably, you can count on 40mm of range while drilling, which permits making five 16mm [10/16-inch] holes 400mm [16 inches] deep in the hardest limestone and eleven holes in the softest. Examples: INTER CLUB (La MALLE): 7 holes; La MANUELE (St. Cezaire): 9 holes. As for the number of anchors…it is more than 250!

6) Charging system

You can use the original charger (see paragraph III), but it is necessary to use a timer to cut it off automatically, and to limit it with a resistor.

How do you determine the charging time?

- If you know how long the drill was running:

Charging time (hours) = Time in use (hours) x amperage of decharging (=10amps) / Amperage of charging (in amps)

- If you don’t know anything about the state of the charge, you can make do by stopping the charge when the temperature of the batteries rises noticeably.

NiCad batteries are a lot more tolerant from the point of view of discharge and prolonged charging than lead batteries. Moreover, they have good voltage over the course of the discharge, because they decrease slightly and fall abruptly at the end of the cycle. In contrast, a lead battery sees its voltage drop regularly, which causes a decrease in the output of the machine.


Published in French at
Caving Technology Website Version 1, 12 October 2000


WB01727_1.gif (697 bytes)
BallBlue.gif (954 bytes) Caving Technology Website home BallBlue.gif (954 bytes) Copyright and Disclaimer BallBlue.gif (954 bytes) Feedback