Bosch Bulldog - Convertiing to SLA batteries, step by step


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Juan Maderita

Trad climber
San Diego, CA
Topic Author's Original Post - May 27, 2008 - 04:11am PT
While at the workbench, converting two more Bosch Bulldogs to SLA (sealed lead acid "gel cells"), I remembered to get out the digital camera. I've been asked numerous times over the past 20 years for step by step directions, and it's difficult to describe without photos.

The Bosch Bulldog is a 24 volt rotary hammer drill, with electro-pneumatic action, and SDS chuck. The models numbers are 11213 and 11213R (reversing). It is no longer in production, having been replaced by the 24v "Annihilator" 11225VSR/11225VSRH. The stripped down Bulldog is light and easy to operate with one hand.

There are problems with the factory NiCad battery packs:
1. They die after a few years and are expensive to replace.
2. Adds to the weight for those of us who like to drill overhead and on lead.
3. NiCads run out of power suddenly. By the time you notice the drill running slowly, it is dead, mid-hole, in seconds.

An inexpensive and reliable solution to the NiCad problem is to convert to a remote battery pack (fanny pack) utilizing two, 12 volt, SLA batteries. SLA batteries are non-spillable, easily recharged, availble in many sizes, and highly reliable. My first set had a 15+ year lifespan! Best of all, they can be purchased for about $20 each.

I recommend using a heavy gauge wire, such as 14 gauge. A five to six foot length of 14/3 (3 strand), or 14/2 if available, makes a good power cord. It should be long enough that when the battery pack is worn on the waist, the drill can be extended overhead.
For those drilling on lead, an extension cord may be used, 35' to 50' in length. 14 gauge is the minimum, 12 gauge (bigger) would be better for less voltage drop. A custom extension cord of 14/4 can be made, with the wires paired together.

Bosch Bulldog, model 11213R, 24v,minus the factory NiCad battery pack. For modification to an SLA battery remote pack, the handle will be trimmed and the aluminum bracket will be discarded.

Remove 3 screws from the handle, rear. Remove 4 screws from the forward assembly. Important: Remove the 4 small screws from the retaining clips over the carbon brushes. Be careful with the delicate spring-loaded brushes.

With the brushes removed, the case will separate. Pull the forward half forward. The aluminum bracket can be pulled forward and then discarded. Do not attempt to separate with the brushes in place. Inspect the armature for cracks/wear. Clean the interior with a soft brush. Upon reassembly, connect the case halves prior to reinstalling the brushes.

Mark the handle for sawing. No need to cut too close. The excess is easily removed with a power sander (or file).

Make the first cut with a fine-tooth saw.

Saw off the excess handle material. Pictured above, both cuts are complete.

Shape the handle. A power sanding disc does the job in seconds.

Unscrew and remove the 2 battery contacts and discard. The new power cord wires will attach to these screws. Note the soft (automotive detailing) brush for cleaning.

For strong and trouble free connections, crimp, then solder. Pictured are "eye" connectors for wiring the power cord to the rotohammer.

Crimp and solder two "eye" connectors. In this particular install, one new wire is red, and the other is black. (To avoid confusing the viewer, there were 2 unused wires in the 14/4 power cord. These were tucked away, underneath the switch).
The wire tie (white plastic "zip-tie") may offer additional pullout strength, if the rotohammer is dropped.

A lanyard should be attached to the rotohammer in case it is dropped. The lanyard may be affixed to the power cord. Thin wire rope ("cable") can be crimped on. Prior to reassembling the handle, drill a properly sized hole for a pop rivet. A hot nail will make a neat hole in the nylon webbing. Pictured is a stainless steel 1/8" rivet. If aluminum is used, go up in size to 3/16". Use a pop rivet washer on the back side. Another lanyard should be attached to the other end of the power cable, near the female connector. That lanyard should be attached to the SLA battery pack, or alternatively, to the extension cord.

Install a female connector to the power cord. This will prevent someone from plugging the rotohammer into a 120VAC outlet and ruining the drill or perhaps starting a fire. Care must be taken to cover the battery pack's male connector. The exposed prongs could easily be shorted by coming in contact with metal.

Pictured is a modified Bosch Bulldog, model 11213R, 24 volt, and SLA (sealed lead acid "Gel Cell") battery pack. This mid-power pack contains two 12v, 4.0 amp-hour SLA batteries. A high-power pack can be made using 7.0Ah batteries and a larger fanny pack. The batteries are duct taped together with an aluminum flat bar for stiffening. The battery package is wrapped for bump protection with closed-cell foam. Unzipping allows acces to two pairs of wires for charging with a 12v smart charger, designed for SLA batteries. Mods to the fanny pack include side exit hole for the power cord, safety buckle on the front should a zipper blow, hammer holster loop, and orange gear loops. The grey adaptor plug is used to protect the exposed "hot" prongs of the male connector. The prongs on the other end of grey adaptor were hack-sawed off. Note the thin wire rope lanyard attached to the handle of the rotohammer.

Here's a link to my info on SLA batteries, fanny packs, chargers:


Social climber
My Inner Nut
May 27, 2008 - 11:05am PT
Thanks for taking the time to post the pictures and description - I just sent this link to a friend who is interested in doing this conversion.

What would be the set-up for a 36-volt Hilti? The series would have to be different in the batteries, right?

Thanks, Erik

Santa Fe, NM
May 27, 2008 - 11:33am PT
E - it would essentially be the same just one more set of crossover wires.

So instead of:

Bat 1 (-) connecting to bat 2 (+) (leads to drill +)
and then on the "other side"
Bat 1 (+) connecting to bat 2 (-) (leads to drill -)

You'd have
Bat 1 (-) connecting to bat 2 (+) connecting to bat 3 (-) (leads to drill -)
and then on the "other side"
Bat 1 (+) connecting to bat 2 (-) connecting to bat 3 (+) (leads to drill +)

rick d

tucson, az
May 27, 2008 - 11:58am PT
i'm with rokjox,

my momma said "electric drills are the devil"

-bobby boucher

Social climber
My Inner Nut
May 27, 2008 - 12:21pm PT
Nature: Thanks

Rokjox: harshing my mellow

Rick: funny
Juan Maderita

Trad climber
San Diego, CA
Topic Author's Reply - Sep 18, 2008 - 06:06am PT
I built two more battery packs and took digital photos this time around.

Aluminum bracket for securing two SLA batteries. 1/8" x 1" flat bar stock. A layer of duct tape is then wrapped over the outside.

Pre-wiring the connectors before installation.

Two 12v, 4.0Ah, SLA batteries, wired in series for 24v output at the male plug. There are two pairs of wires for charging with a 12v SLA smart charger. The gray adaptor (w/ male prongs cut off) is a protective cover for the male plug to prevent a short during transport.

The package is securely wrapped in duct tape to prevent connectors from pulling loose and shorted circuits.

Wrapped in protective closed cell foam. I hope the wife doesn't notice that her yoga mat is 2' shorter...

The completed SLA battery pack for a 24v Bosch rotohammer.
Buckle added in case the zipper blows. Hammer holster and gear loops added to the waist belt.


Trad climber
Boulder Colorado
Sep 18, 2008 - 11:55am PT
nice work - thanks for posting.

Trad climber
Anchorage, AK
Sep 18, 2008 - 12:45pm PT
Sweet! I need to pick one up off of flea bay or a hilti te-10A. My ryobi has served well over the years, but it's just too heavy and bulky for lead bolting.


Sport climber
White Salmon, WA
Sep 18, 2008 - 01:43pm PT
I put together a pack with two 12V 3.4AH SLAs.

It runs for less than 10 minutes, so only 6 3.5" holes..

I had much higher expecations.

Do you know how many amps the 24V bosch annilator pulls?

Thanks for the details!
Juan Maderita

Trad climber
San Diego, CA
Topic Author's Reply - Sep 18, 2008 - 02:10pm PT
The Bosch "Annihilator" now has the BAT021 which is 3.0Ah.
I don't know how much amperage the drill motor draws, but that probably varies somewhat according to the load (small vs. large bits, etc.)
In good granite my Bulldog drills a 3/8" x 4" hole in 45 to 50 seconds. 10 minutes of drilling time should give you about a dozen holes. There are variables, particularly the hardness of the rock. Are you drilling in granite?

You might try sharpening the SDS bit carbide tip. Yes, even on a brand new bit. Use a body grinder with an aluminum oxide sanding disc (the reddish-brown color). Grind a more acute angle. No need to be precise as these bits cut by percussion.


Mountain climber
The Deep Woods
Sep 18, 2008 - 02:39pm PT

This is great info.

Can you hand-sharpen SDS bits? I have the same problem - only getting about 10-12 10mm 3.5"deep holes per charge with my new Annihilator even with a new bit.

Trad climber
Anchorage, AK
Sep 18, 2008 - 02:57pm PT
You can use a course diamond sharpening stone to hand sharpen carbide bits. The best option is a bench grinder with the green wheels for carbide.


Trad climber
Grand Junction, CO
Sep 18, 2008 - 03:35pm PT
I have a modified Bosch Bulldog up for sale. Make me an offer and I will probably take it! Email me:

Juan Maderita

Trad climber
San Diego, CA
Topic Author's Reply - Sep 18, 2008 - 04:50pm PT
Here are some troubleshooting ideas which might help:

More detail on sharpening your SDS bits:
A bit is worn out when the width of the carbide tip equals the diameter of the shaft. Notice that the carbide tip on a new bit is slightly wider. That prevents binding/drag. The carbide tip width can be tested by touch with a fingernail. A worn out bit will cause drag and use up your batteries in a hurry. I usually sharpen bits 3 or 4 times before retiring.

Battery condition:
Are you using a battery charger specifically designed for SLA batteries? Use an SLA "smart" charger, output 500mA to 1000mA. You might use a volt meter to test that they are holding a charge. Conduct that test after charging, and then hours or days later due to the "surface charge" phenomena. Better yet, use an SLA load tester to check their condition, such as Triplett's "Chek-A-Cell".

Is your primary wire 14 gauge or bigger (smaller numerically)?
If using an extension cord, is it 50' or less? Longer and the voltage drop would be a problem. An extension cord must be heavy enough to carry the load without voltage drop. 12 gauge or bigger would work. If you make an extension cord of 14/4 (14 guage, 4 strand wire), you could pair up the wires (2 positive, and 2 negative).

Condition of the Bosch:
Check your carbon brushes on the motor. They are designed for ease of replacement. If your drill has had a lot of use, $15 will freshen it up. If you have it apart, you could check the condition of the armature. The grooves on the rotating surface where the brushes make contact should be nonexistent to minimal. Clean off dust and any grease from the armature.
Clean and grease the SDS chuck.

Operating the drill:
Are you applying too much forward pressure when drilling? Use only enough force for the bit to hammer and cut into the rock. More pressure is bad for the Bosch and can increase the drilling time. If I were to guess, 10lbs. of pressure is about right.

Remove the dust/debris 2 or 3 times during the course of drilling a hole. If drilling on lead, the quickest way to accomplish that is with a quick outward pull on the bit while drilling.

Hope you can increase the performance with these tips.

Sport climber
White Salmon, WA
Sep 18, 2008 - 05:09pm PT
Thanks, I'll certainly check the brushes, and file the bit.
Juan Maderita

Trad climber
San Diego, CA
Topic Author's Reply - Sep 18, 2008 - 05:11pm PT
"Can you hand-sharpen SDS bits?"

I'm talking about seriously grinding off some material to change the angle of the faces for the initial sharpening. That would take about three days by hand! These tips take a pounding. It's not like sharpening a knife.
A better strategy would be to use a power grinder at home/shop and bring several SDS bits on your trip. I swap out the bits every 6 holes when trying to conserve battery power.

"I have the same problem - only getting about 10-12 10mm 3.5"deep holes per charge with my new Annihilator even with a new bit."

I don't think you have the same problem. With the model# BAT021, 3.0Ah ni-cad battery, that sounds about right. That is roughly the equivalent of slightly smaller but longer holes @ 3/8" x 4".
I'm guessing that you are within about 25% of optimal drilling. Try sharpening your bits and cleaning the holes while drilling.
Again, watch for excessive forward pressure while drilling.

Ni-cads go bad over time, whether you use them or not. If they are not holding a full charge, then it's time to replace them.
The BAT021 should have the convenient built-in LED charge indicator lights.
Check eBay for some good deals, $110, way below retail. Or have your ni-cad pack rebuilt. There's a company which sells rebuilding service through eBay.

Big Wall climber
Sep 18, 2008 - 06:00pm PT
very cool stuff guys! Love the mods, Juan!

Trad climber
Anchorage, AK
Oct 3, 2008 - 06:13pm PT
They say power corrupts, and I'd rather talk about corrupt climbers than politicians.

Trad climber
Las Vegas
Sep 3, 2010 - 01:20pm PT

Juan. Question:
In your suggested wiring...

You take the power from the neg/positive poles of two separate batteries. I understand this is part and parcel to the 'series' connection, as well you have a wire connecting the other two poles of the batteries. This second (series connection) wire, looks like it remains connecting the two separate battery terminals at all times? It does not hinder the recharging process ?
Somewhere I read on the Taco (can't re find thread) someone mentioning similar wiring but with a toggle as cut off between the two batteries for the charging process.

Needed? No Need?

Juan Maderita

Trad climber
San Diego, CA
Topic Author's Reply - Sep 3, 2010 - 02:36pm PT
Wire the batteries as shown on that first page, below my original post.
The 2 pairs of wires (red = positive, black = negative) allow for charging each battery separately with a 12V "smart" charger. No need for a toggle switch. Tape it up securely after you test your system. You should not have to mess with the wiring again, ever.
There are probably other ways to wire the pack, but this method is simple and proven.

Batteries are best charged individually, particularly when one battery is in better condition than the other, such as age, or there are unseen internal resistance differences.

A couple suggested improvements:
Might be a good idea to add an inline fuse holder on the 24V ouput cord.
Make it outside of the taped up battery pack, but inside the fanny pack, keeping it accessible for fuse changes. Keep extra fuses in the pack.
Use a quality, heavy duty fuse holder with thick wire (10 gauge?). Not sure what fuse will be needed, at least 20A, maybe 30A or more. I haven't found regular inline automotive fuses in a size above 30A. Try it out under a load such as a deep hole with a 1/2" bit. I haven't used fuses on my packs, so you'll have to let me know how it works.

On the charging pairs of wires, use male connectors (plastic covered type, as shown) on the ends of the wires which will protrude from the taped up pack. I used some female connectors because that's what I had in the garage. It's a nuisance to connect the charger's alligator clips to the female connectors.

Have you been using your Bosch, or just now getting around to it?
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