CH-1092 Belmont, Switzerland
Tel +41 21 728-6156, Fax 728-6157
Email info@didel.com, www.didel.com
File www.didel.com/slow/magnets/Bahoma.doc
Battery Holder using Magnets (BaHoMa system)
November 2004 update
BaHoMa (Battery Holding with Magnets) was first defined in 2002, used on the Celine home flyer
and proposed as a standard. Larry Park (SmartTools USA) and Bud Kays (Plantraco Canada)
had good ideas to improve the technology and Plantraco just announced his very nice blimp (see
http://www.plantraco.com/product_microblimp.html) powered by a Bahoma LiPoly. Unhappily they
changed the polarity. Since it makes no sense to have two polarity standards, we had to change.
This document describes the Plantraco polarity, and we insist that all designs must be made
compatible. There are enough incompatible things in this field. Thanks in advance to the first
Bahoma users to make the change, or to clearly mark their receivers and Lipo as being
incompatible.
On a very slow flyer, it is very convenient to be able to hold the battery to the plane with magnets
while establishing the electrical contact at the same time. This adds two magnets to the plane, and
two on the battery pack, but is saves the JST or similar connector and wires. It is really worth it
since the battery is inserted and removed in a snap (no on/off switch required), and the battery
separates automatically from the plane in case of a harsh landing.
The 2004 solution is easier to implement thanks to the use of gold plated magnets, that allow
direct contact instead of the thin CuBz contacting wires. The idea came simultaneously from Larry
Park (www.smarttoolsinc.com) and Bud Kay (www.plantraco.com) and both companies will
support Bahoma components (magnets, batteries, charger).
The essential trick is to have the magnets in the oposite direction, so if the battery is mispositioned, the magnets will repel and the battery will refuse to go in the wrong position. But if the
magnets are not installed in the correct direction, the receiver may get a reverse voltage. It is thus
important to carefully follow the instructions below.
Standardization
In addition to the polarity of magnets and the electrical polarity, the distance between the magnets
must be normalized. Anybody will then be able to borrow a friend's pack, and just put the battery
on a Bahoma system charger (e.g. the Li-Cell charger from SmartTools). We are concerned only
about less than 200 mAh LiPoly batteries weighting up to 4 grams. The BH10 standard specifies
10mm between magnets, and 1.5 mm diameter magnets. 2mm magnets or more are possible and
compatible. Dia1.5x1.2mm magnets (0.01g each) have an attracting force greater than 30 grams,
hence 60 grams for both. The initial Bahoma proposal required larger magnets, since the
contacting wires were adding some distance between magnets. Gold plated magnets guarantee a
better contact with smaller magnets, but the magnet size is not critical.
Magnets can be glued on the battery tabs and on to a drilled plywood piece on which the leads will
also be glued with a conductive elastomer adhesive. Soldering is usually not possible, since the
magnets loose all their magnetization above 160 degrees. With high temperature magnets (grade
30, > 180°) and low temperature solder (< 135°) it is however possible with an adequate soldering
iron. Conductive glues are expensive and the result is difficult to guarantee if not done industrially.
DIDEL has manufactured small precision circuit boards and press-fit the magnets to create the
Bapi (for “Bahoma bracket piece”). Weight with two magnets is 0.082 grams, hence 0.16 gram for
the pair, which is less than a connector or switch.
The Bapi circuits have two tabs on which solder can be applied for connection to the battery tabs
or the wires toward the receiver. When soldering, the heat must not travel toward the magnet. The
Bapi-B (on the battery side) and the Bapi-M (on the model side) are not identical due to magnet
orientation, like male and female power plugs.
Connection to the battery
Connection to the plane
When the Bapi-B is soldered, if one looks to the When the Bapi-M is soldered and applied
battery hanging on the plane, the positive tab is on the plane, the positive tab is right and
right and the visible pole is the north pole.
the visible pole is the north pole
Price for one Bapi circuits with two press-fitted magnets is 1.10 CHF, about 0.9 USD or 0.75 €.
Specify Bapi-B (for battery side) or Bapi-M (for model side).
How to do it without the Bapi pieces?
If you have conductive glue, use the SmartTools laser-cut plywood brackets or do similar pieces to
guarantee the 10 mm spacing between the magnets. Do your best to firmly glue the magnets on to
the battery or wires. We have had bad experience with expensive conductive elastomer. Be sure
you handle and use that technology professionnaly.
The initial Bahoma proposal was using crossed 0.15mm CuBz wire between larger 2mm magnets
(due to the distance), see www.didel.com/slow/magnets/Bahoma03.doc. That solution is still
excellent and compatible, but the Bapi magnets must be well aligned on the 0.15mm wire.
How to recognize the North pole?
If you have a compass, it is easy - the south pole of your magnet will
attract the north pole side of the compass. Paint the north pole of the
magnet red and glue it at the end of a balsa rod. It will be your reference
piece for testing polarities.
If you do not have a compass, build it! Glue two magnets with the same orientation on a stick of
balsa. Let it float on a glass of water, and put red paint on the North direction.
Never heat the magnets!
The major problem is to not heat the
magnets above 150 degrees, by conductivity
or by accident. The heat propagates through
the copper PC board when you solder, or the
magnet suddenly attracts the soldering iron
tip. A small touch and the magnet is useless
and must be changed!
Stick the Bapi on a steel plate, or, if a short is possible, use 4mm magnets or bolts self-stuck on
each side of the 1.5mm magnets to slow down the heating when soldering wires or batteries. Steel
nuts could be used in place of magnets. Soldering must be quick, and as usual, the PC board and
the wires should be tinned before they are put together. Of course with batteries the danger is to
short-circuit them, so be careful with any metal pieces that can be attracted. With the bolt solution
pictured next, it is a good idea to put smaller bolts or a washer on top, os if the soldering iron is
attracted by the magnet, the the tip touches the bolt or washer instead of the magnet.
Preparing a battery
Detailed explanations using the
Bapi pieces can be found at
www.didel.com/slow/magnets/Lip
olyMag.doc. The picture on the
left left shows several results.
The non-contact side of the Bapi
must be protected to avoid shorts
on this side. Use soft balsa, paper
labels or nail paint.
It is good to have a Bapi-B with
two wires going to your usual
battery connector, and a Bapi-R
with wires going to your preferred
voltmeter, in order to check LiPolys.
Use cellophane tape as documented to protect the contact side and to allow for non-powered glide
testing by leaving the tape between the Bapi-B and Bapi-M.
Plantraco protects the terminals of the Bahoma cells with a
plastic safety guard. Their magnets are larger due to a
higher Curie temperature that allows the to attach the
magnets with low temperature solder.
See http://www.plantraco.com/product_microblimp.html
The Bapi-Rec pieces can used with
any battery charger. The charger
from http://www.smarttoolsinc.com/
has already been adapted with 10mm
contact points for glued magnets or a
Bapi-B add-on.
We hope to see in the future other
Bahoma chargers, and even LiPoly
cells
with
built-in
gold-plated
magnets.
jdn 040624/040821/us041010