Bahoma with protection on Lipo batteries (2003 ... - Didel.com

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www.didel.com/vole/LipoProtection.pdf
<strong>Bahoma</strong> <strong>with</strong> <strong>protection</strong> on Lipo batteries (2003 document to be updated)
Lipo construction
Small Lipos are built in a very similar way.
The envelope is folded. The negative tab is made of Nickel, the positive tab is
Aluminium.
In order to have the positive tab solderable, there are two solutions
- A Nickel tab is soldered on the aluminium. The Al part of the tab is usually folded
and protected <strong>with</strong> a sticker, so the user sees only the nickel tab
- A copper mesh surround the alu tab and is probably glued <strong>with</strong> some conductive
glue.
On the batteries I have, the + is always on the same side, except for a 0.5g lipo.
<strong>Bahoma</strong>
<strong>Bahoma</strong> (Battery Hold <strong>with</strong> Magnets) uses gold plated magnets in order to both hold
the battery on the equipment and establish the contact. Inverted polarization of the
magnets avoid an polarization inversion.
There are several solutions to connect magnets to the tabs
1- The magnets are soldered on a PCB, and the tab soldered on the othre side
(Plantraco). High temperature magnets and low temperature solder (150°)
have to be used. The N35 magnets are not as strong as low temperature
magnets
2- The magnets are press-inserted inside holes on the PCB (Didel). The problem
is that drilling and plating the holes is not very precise. PCB manufacturer
must guarantee the diameter, but it is not sufficient. On 1.5mm specified holes,
Didel inserts 1.50, 1,52 or 1.54 diameter magnets, depending on an initial
measure.
3- The magnets are directly soldered on the tabs, and positionned by a plastic
piece. That solution could be the best, but soldering techniques have to be
developed together <strong>with</strong> a manufacturer.
4- The magnets have a hole and are manitained by a rivet. This seems to be
inadequate for the small magnets we consider.

Magnet spacing and position on the plane
<strong>Bahoma</strong> has two standard distance for magnets: 10mm plus a 5mm defined later by
Plantraco (bad idea that complicates the life of the users and designers).
The 5mm was required to fit inside the narrow 30 mAh Lipo from ABF and is maybe a
good option for the day 10mAh Lipos are available for 2 grams planes. Other 30 mAh
Lipos are larger (13mm) and accommodate the 10mm standard.
The receiver PCB is tailored to the magnet spacing. The 5mm
allows for a smaller circuits, narrower but not significantly
lighter.
The Lipo hang on the circuit and there are three solutions
1- The magnets are on the top of the receiver PCB and the
Lipo lies flat against the circuit (Plantraco). This is
compact, but not convenient to position and remove the
Lipo.
2- The magnets are on the bottom of the PCB. The Lipo
hangs below, and is easily accessible.
3- The magnets are at mid-height of the PCB. This may only
have sens if there are two rather independent functionnal
blocks.
Didel Mip21
Plantraco
Didel-IGI
Amigo
Didel prefers a hanging Lipo, easy to catch. The Amigo receiver needed to have the
IR receiver at the bottom of the circuit, but the short circuit allows for an easy catch.
Didel insists also that a LED is visible and blinks when the Lipo is inserted. A blue
LED (expensive) <strong>with</strong> its 3.2V drop would indicate also if the battery is good enough.
Protection options
The problem is to avoid the magnets touching a conductive surface and being
destroyed by the short-cut. This is not dangerous <strong>with</strong> a few grams Lipo, they just get
hot (less than 100 degrees).

Plantraco uses patented guard rings which provide the best <strong>protection</strong>. The drawback
of the rings is you cannot slide the Lipo to remove it (not easy pull down if the Lipo
hangs against a plane).
The other solution, not proposed in Plantraco's patent, is to have a protruding post
between the magnets, so both magnets cannot touch simultaneously a flat
conductive surface. This reduces considerably the risk for a shortcut. The bad case is
a razor blade <strong>with</strong> its middle slot.
A special plastic piece <strong>with</strong> a bumper is glued. It holds the PCB at the best position
and covers the tabs going out of the Lipo.
The proto has been built on a
4g Lipo, for making the
machining of the <strong>protection</strong>
easier. 1 gram Lipos will just
need a smaller geometry.
Soldering is made at the tip of the tabs, cut at the correct length. Then the tabs are
folded and the plastic piece inserted.
If the magnets are soldered directly on the tabs, the PCB is not required, and the
plastic <strong>protection</strong> is designed to hold the tabs in place. Precision molding may be
required.
Planning
Narada is equipped <strong>with</strong> spot soldering and test the soldering of magnets as soon
they get magnets (will be supplied by Antaida ShenZhen).
Tests and discussion to find the best manufacturing process will take place early March.
Plastic pieces will be inserted in the Amigo new molds and be ready end of March,
when hopefully the best solution for soldering magnets has been found.