TIP TOP LIPOS – PART 1

Danny Fenton looks at charging and temperature management of LiPo batteries.

For this coming flying season, if indeed there is one, I will be competing in ‘Flying Only’ with two airframes, a Black Horse 85” Chipmunk and a Seagull 80” Chipmunk. The Black Horse takes a pair of 5S LiPo packs, giving 10,000 mAh and the Seagull a single 5S-5000.

The Blackhorse does not need the second pack, other than for nose weight, but the extra capacity does mean the cells are not being worked hard as each pack is only having to deliver around 25A, so they can be cheap and cheerful.

The single 5S LiPo in the Seagull, however, is operating at around 50A so a cheap battery gives a reduced performance.

When I looked at my elderly chargers and my three-year-old packs, which were cheap Hobby King items to begin with, I thought I had better have a serious think about better looking after my LiPos.

CELL INBALANCE

The first thing I looked at was my existing packs and I could see that cell five was misbehaving on all my packs. I had not noticed this before and decided that perhaps upgrading my chargers would be a good start. My thought was that something was not balancing quite as it should. My pair of I-charger 208s have done sterling work but are perhaps a bit behind the technology curve.

I had a good look around and the ISDT range were pointed out to me as having lots of features in a small package. They looked nice and compact, but with serious charging ability. In fact, ISDT produce some nice kit and are clearly pushing into the market quite aggressively, with great products at keen prices. And no, I do not have shares!

I started with an ISDT6 (6 cell) and was so impressed that I then bought the 8-cell touchscreen version, the ISDT8.

The two chargers worked better than the older models, but I think the damage, due to a potentially faulty balancer, had meant the batteries were past their best. This really did show up in competition flying where differences were easily seen between the packs.

The situation was not helped by the batteries not always being stored at a storage charge and in sometimes a cold or hot garage. But the problems associated with storage environment we will look at later.

NEW PACKS


To this end I thought I would break my own ‘norm’ and buy two serious packs. There are a few vendors that make 5S-5000 packs at around the £100 – £150 price range. I had heard great things about George Worley’s 4-Max Purple Power cells, so I decided to give them a go. These were duly ordered and arrived the next day; even if you ignore Covid, that is impressive!

As a direct contrast, I also spotted a 5S cell from a manufacturer I had never heard of, of the same size pack as the Purples, which was just £30 and supplied from a UK warehouse. So, I thought it might be interesting to see how they compare over time if looked after in the same way?

I have flown electric models for some time, even before LiPo packs were really on the scene, being a keen user of the A123 cells removed from DeWalt power tools. And I still have models, some 12 years later, that are flying on the same cells; they have lost a little capacity, but I cannot say I have kept any LiPos alive that long, and certainly not useable.

KEEP ‘EM WARM

To make sure I was armed with good information, I reached out to George at 4-Max and asked for his advice on the best way to ensure the new packs lasted.

George provides a detailed data sheet on charging and storage, but I asked if he had any additional thoughts on storage temperature, especially when winter flying and storing the packs in a cold garage:

“When you take a battery that is charged in the warm and then taken to a cold flying field and left in the open for an hour or so you should notice that the overall pack voltage has dropped compared to when it was in the warm.
If you have a really cold battery and an amp hungry model, such as an EDF jet, you may experience a lack of power and possibly the motor surging.

The motor surges because the battery chemistry does not like the cold (it’s the same for all types of batteries.) It cannot supply the current needed, plus the voltage drops a lot more under load then would happen with a warm battery.

The extra drop of voltage causes the ESC to think that the battery is depleted.

The ESC will then cut power to the motor, thinking it is protecting the battery from over discharge.

When the amps are reduced, the voltage of the battery will rise again and so the ESC then restores the power and so the cycle is repeated until the internal temperature of the battery rises due to it being used.

The ESC no longer thinks the battery is empty and the surging will then stop.

So, keep your packs warm; they perform better. But do not put them in your pockets!”

CHARGING BOX

I will come on to storage temperature later, but first I have always been unhappy with balancing chargers on my battery ammo boxes, with wires trailing everywhere – you know the scene.

I wanted to eliminate the inevitable broken connections and even the chance of wiring errors by having a neat charging case dedicated to field charging. I want to show how I made my charging case in an effort to make life easier at the field, and specifically in the heat of competition.

I had an old transmitter case designed to hold two handsets and thought that there should be enough landscape into which I could install two ISDT8 units. I would need to provide cooling, but it looked doable. I retained the smaller ISDT6 as a separate unit that could be driven by the case but positioned alongside an aircraft for charging onboard LiFe Rx packs.

I ordered some 6mm MDF from an eBay supplier, who even supplied it cut to size. I marked out the possible position of the chargers and balance leads. I did not want to make something where everything was crammed into a small area, but something that gave space around the charging packs to access the plugs and wires. Planning the layout was probably the single most difficult bit and it did take some time to get how I wanted it. This probably would not suit all but for my 5S-5000 packs it suits me. Time will tell.

I made and installed a maple frame into the case to support the panel. The rails are attached with screws from the outside. The base of the case was lined with foam rubber and I used some spare foam from the sides to stop the chargers sliding around in their cradles. Two holes were cut in each end of the case for two pairs of 40mm DC fans. These, I hoped, would create sufficient airflow to keep the chargers cool. Some nice chrome grilles should stop fingers and objects getting caught in the fans.

I chain drilled holes to cut out the MDF and then cleaned up the edges with a variety of hand files and PermaGrit tools. I also bought several packs of 4mm ‘banana’ type connectors that could be surface mounted to allow the packs to be connected to the chargers. And to bring power into the case.

I also bought a couple of balance charger ports that would give 2S to 8S connectivity.

TERMINAL BLOCKS

I discovered solid metal terminal strips used by electricians that I had never seen before. They provide a perfect way to make a busbar of sorts. The blocks are very firmly secured under the upper board. These allowed me to co-ordinate all the wiring and keeps it away from fingers and anything that may drop into the case.

The external supply to the case feeds these two blocks and then smaller gauge wires feed the two chargers and the case cooling fans.

Once I was happy with the fit of everything, I used some carbon effect vinyl automotive shrink wrap to cover the top board. The balance ports were superglued to the vinyl. I was concerned that they might loosen as the JST balance connectors can be a snug fit to undo, but so far, they have stayed stuck fast. There are screw holes on the balance port boards but I was concerned about shorting the tracks, so I opted to super glue them first and see how they hold up.

There is plenty of space on the case to charge two 5S packs simultaneously. And as I only ever charge at 1C the load on everything is kept sensible. I made up a special supply lead with clips at one end for the leisure battery and 4mm gold at the other to fit the banana plugs. You must watch the length of cable; too long and the voltage drop will be high.

You do have to be careful with polarity and I might substitute a surface mount XT-90 connector to ensure polarity is not an issue.

Continued in Part 2…