HELICYCLE.ORG – BUILDER LOG
BATTERIES AND BATTERY BOX
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There is a fundamental problem with the stock Helicycle electrical system. The starter draws so
much current - approximately 1100 peak Amps during an engine start, that it causes the battery
terminal voltage to drop to a level that is too low to support the engine electrical components
such as the igniter and start fuel solenoid -- both required to start the engine. More on this
later…
A popular battery choice has been to use two Odyssey PC680 lead acid batteries in parallel.
That’s what I used on my first Helicycle. These batteries are high quality and very reliably. They
are also relatively inexpensive, but also very heavy. Here’s what we’re up against:
This high current consumption is not the problem, but it is the cause of the problem. See the
next graph…
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BATTERIES AND BATTERY BOX
This is the problem (graph above.) All batteries have internal resistance. What that means is
that, under load, the terminal voltage will drop. Ohm’s law is very simple: E=IR. ‘E’ is voltage,
‘I’ is current, and ‘R’ is the battery internal resistance. In the case of the Odyssey PC680 the
published internal resistance is .007 Ohms, so two 680’s in parallel would be half that or .0035
Ohms. Doing the math we multiply 1100 by .0035 and we get 3.85 Volts. That’s the voltage
that is lost inside the battery due to the internal resistance. Subtracting that from the 12 Volts
and we end up with 8.15 Volts which is pretty much what the graph shows. It’s hard to be
precise since the age of the battery, the state of charge, and the battery chemistry will all affect
the no-load voltage. In my test the batteries had just come off of trickle charge so they present
a misleading no-load voltage. Lead acid batteries need to sit for at least 24 hours after charging
or discharging to reach equilibrium. The critical data here is not the no-load voltage, but the
voltage under the high current load imposed by the engine start.
To get around this problem there are a number of possible solutions:
1) Drop the starter, hit the igniter and start fuel, wait for the engine to light, and then get
back on the starter (this is what many folks do.)
2) Automatically switch to an alternate voltage source during engine start (that was my
solution.)
3) Get batteries with less internal resistance so the voltage drop under high current loads
is less.
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BATTERIES AND BATTERY BOX
Taking these in order, option-1 works but I find it too complicated and slightly hokey. Option-2
also works but it results in a more complicated electrical system which means less reliability, at
least in theory, and more potential single-point failure modes. Option-3 was a non-starter up
to now because of the increase in weight that even larger lead acid batteries would entail.
But now there is an alternative to lead acid battery technology – Lithium Iron Phosphate
(LiFePO.) There are a number of other lithium battery chemistries and some are borderline
unstable and unsuitable for manned aircraft. Even extremely high quality lithium batteries
need to be viewed with caution. Look at what happened to the Boeing Dreamliner. It was
sidelined for months after a series of lithium battery fires.
I’m currently deciding what if any changes to make to the electrical system in my new Helicycle.
As I mentioned, the Odyssey PC680’s have been a very stable and reliable power source. But
the internal resistance and heavy weight are troubling…
While I work to resolve this issue I am pressing ahead with a box to hold the batteries. It will be
patterned after the one I made for my first Helicycle and will mount directly behind the upper
fuel tank.
I’ve been communicating with the chief engineer at EarthX, the maker of a range of LiFoPO
batteries. Their batteries contain cell balancing circuitry as well as protective circuitry for overvoltage and over-current protection. This produces a lithium replacement for a vehicular lead
acid battery. These batteries can be charged in the normal way with any automotive alternator
or a modern lead acid battery charger.
One failure mode of this protective circuitry is a unexpected disconnect. That would be a
serious failure if it happened in flight so two of their batteries should always be used in parallel
for redundancy. The good news is that they have a battery with similar characteristics to the
Odyssey PC680 and it’s considerably lighter. Unfortunately it has a similar internal resistance
to the PC680 so even two in parallel would suffer from the same voltage drop during an engine
start.
They do make a more powerful battery that might provide enough voltage (two in parallel) to
allow a normal engine start without the need to pulse the starter or switch to an alternate
power source, but unfortunately two of these will set you back about $1450. Another concern
is the higher likelihood of a battery fire. That’s an unknown but the company does warn to
always keep the battery away from flammable material and never leave it unattended when
charging. That leaves me undecided between the Odyssey PC680’s and a pair of EarthX ETX36
LiFoPO batteries.
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BATTERIES AND BATTERY BOX
Meanwhile I’ve been working on the battery mounts which I’m CNC machining. The right side
plate will mount to the upper frame tube with Adel clamps and house the master switch and
other components associated with the DC power. Machining this piece was about a five hour
job, not including the time it took to CAD the part and setup the tool paths, feeds, and speeds
for each operation.
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BATTERIES AND BATTERY BOX
Here’s the right hand side plate with the components mounted. At the lower right is my DPDT
master switch. To the left is the 50-Amp ammeter shunt. Left of that is a fuse and fuse holder.
In the center at the top is a 275-Amp, 400-Volt rectifier, and a 30-Amp SPDT relay sits in the
upper right. The battery box will mount above these components. I’ve fabricated three copper
bus to interconnect components that are adjacent to each other. That’s a lot cleaner than
using wire for these very short lengths. I also have one #8 wire jumper. There are a few
additional internal interconnects to the relay, then everything else will connect to external
items such as the starter motor, battery, alternator, and +13 volt bus.
The Master Switch will have its cover installed once this is completed, and an additional cover
will protect all of these components and extend across the entire area.
The rectifier takes some explaining… When I press the starter button on my Infinity cyclic grip
is will energize the 30-Amp relay. The relay will engage the starter solenoid and switch my
essential DC bus to a secondary power source. That gets me around the momentary voltage
sag caused by the starter’s high current consumption. If that relay failed during an engine start
it would be annoying, but if it failed in flight it could get me killed. A failure in flight is
extremely unlikely since the current flow in flight is well within its ratings, but it is still a
concern. That’s where the rectifier comes in. It’s in parallel with the relay’s normally closed
contacts. As long as one of those two devices is functional that potential failure mode is greatly
reduced.
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Immediately after an engine start, when the normally closed contacts mate, there will be a
current surge as the batteries are again connected to my secondary power source and the
voltages equalize. My analysis tells me that the current surge will be reasonably small, but it
would be very difficult to pin that value down, so I opted for a humungous rectifier to be safe.
Here’s the bottom of the rectifier mount
that I fabricated out of high density
polyethylene. I potted the counterbored
mounting holes in the bottom with
epoxy to prevent any sneak paths to
ground as both sides of this rectifier are
hot to ground. The bus sticking out the
side goes to the fuse.
The frame slopes inward towards the
transmission so the frame tubes that
support the battery box are not parallel.
That means that the support members
that wil come off both sides are angled by
about six and a half degrees acording to
my measurements. So the mounting
bolts that will secure the batteries to the
side plates need to be angled too. I’ll be
using three AN4 bolts on each side to
mount those support pieces. Those in tern will mount the battery box which is a stock item
from Odyssey. If I opt for lithium batteries I’ll have to make adjustments for their slightly
smaller size. I did these holes and counterbored the washer flats by hand using my mill. It
would be too easy to screw this up using the Tormach. One wrong command and it would tear
this plate up. The tricky part would be alignment of the X and Y axis since I had to work from
both sides and mount the plate at an angle to the bed.
I’ll add to this as I go…
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