UM19vB and C - SLA Battery Units

The UM19vB and C are almost identical sealed  lead acid battery units designed to supply a large amount of power within a portable unit for use with the UM11/12 motion control timelapse camera rig in remote areas. These new battery units are a modified and improved version of the UM19vA battery unit.

My original battery the, UM19vA, contains two 18Ah 12v sealed lead acid (SLA) batteries. These two batteries were configured into a +12 and -12 volt supply. The reason to create a +/-12vdc supply was the need to have both a 24vdc supply for the stepper motor controllers and also 12v for the camera's power and the logic circuits. So the logic at the time was to have two 12v batteries in series. I did have an earlier unit which used a linear regulator to convert the 24vdc down to 8v for the camera which wasted a serious amount of power via heat.

This turned out to be more work than was necessary.  As I learnt more, I realised that a simpler approach would be to convert my 12v supply up to 24vdc via a step up buck converter. I could have also done something similar at the other end inside the dolly where I could have converted the supplied 12vdc to 24vdc for the step controllers but I felt that I should leave the heavy current loading at the shorter end within the battery units.

The original UM19vA battery was a complicated beast. It contained several relays which were used to configure the batteries for it's two states - the 24v output state and the charge state. In the charge state the two batteries are separated so that a common ground battery charger could be connected. In the 24vdc state the two batteries are placed in series making one of negative / ground connections NOT common.

These relays, 8 in total, needed 500mA when all were activated whereas the new units which use the step up converter only has two relays. The two relays in the version B and C simply disconnect the 12vdc batteries from the buck converter so that when they are being charged they don't have load which upsets the battery charger. During operation the two batteries are  in parallel. The reduction in relays certain saved a considerable amount of capacity when we are operating the unit over 12 hours.

To further improve the power efficiency of the new units I learnt a method of driving relays with pulse width modulated power.  This power is essentially only on for 1/3 of a cycle 800hz but the frequency is pulsing so fast the relay doesn't have time to turn off.  This in turn reduced their power on time thus reducing their current load. I have found that I can reduce their individual  power usage from 75mA down to around 22mA without any operational compromise. The circuit needed a couple of large capacitors to help smooth the power line. There has been no signs of power instability caused by this relay control method. The relay coils oscillate at an audible level and I suppose there is a chance the relays may not last as long but the power savings are very welcome. Another obvious improvement could have been a relay with more poles to reduce the relay count to one but the only other relay I had was a larger and uses more power.

Once I decided to use a 12vdc to 24vdc buck converter my initial tests showed me draining an 18Ah battery faster than I thought it would. The reason was that I was needing about 1.5A at 24vdc which translated into 3.2A at 12vdc from the battery.  The solution was to put the two batteries in parallel giving me a 36Ah battery which reacted better to the load placed upon it. Having the two batteries in parallel game me more than double the run time. The older UM19vA has also been converted to this arrangement although this battery unit has two batteries which were purchased several years apart which isn't ideal as the older battery will tend to discharge the newer one.

The 24vdc outlet on these batteries has a 3 pin plug as they originally deliver +12, common and -12v. Along with this change to 24vdc, I have used the now spare pin on the 3 pin connector to be a data line. This data line is used to indicate the state of the battery system. The UM17 battery switcher uses this data pin in its management. The two states of this pin are high (5v) when power is available and low (0v) when the power is going to disconnected. So when one of these UM19 battery units hits the depleted voltage threshold it will turn this pin low and switch off totally in 10 seconds. The UM17 battery switcher will react and change it's input to the other battery before the depleted battery turns off. The UM19 batteries have a simple voltage divider which is used to measure the voltage. Once the voltage gets below 10.5v they go into shutdown state.

The UM12 dolly has also been modified to accept this change of supply voltage and configuration. In reality this unit was the main reason for changing as I destroyed the expensive microprocessor due to a misplaced ground connection. Having the unit running from a centre ground caused some confusion in my wiring. Some parts ran from ground and +12v whereas other parts ran as 24vdc which in reality was -12 and +12v. Now that I had discovered an efficient way of converting 24vdc into 5v via a cheap buck converter, I could easily change the system over to a common ground unit. Hence less chances of loosing the magic smoke.

Another decision was made to not include any lighting control outputs on the version B and C batteries. I have designed and partially build another two lighting units, the UM27. UM27vA is self contained with a 9Ah battery and has three lighting control outputs. The case also has space to store the UM27vB unit, lights and cables. The UM27vB is much smaller but essentially the same. This relies on an external power supply like the UM26 video monitor power supply. These lighting units do not need lots of power and it made sense not to rely on using the UM19 battery units for the lights and to utilize other power source already in existence within other units. It also allows for the lighting control and power to be as close to the light as possible hence the removal of long cables. That said, I've added an extra 3 pin connector should I want to run lights from one of the batteries. I've added an extra 3 pin connector as well should I want to run lights from one of the batteries.

The case design of the UM19vB and C is slightly different for a couple of reasons. I decided that having a front panel that covered the whole case was a waste of acrylic plus the smaller front panel had the advantage of direct access to the battery terminals with the need to remove the front panel in case we wanted to run power directly from the batteries. Flexibility is a good thing.

The front panel has charger inputs for both batteries, fuse for each battery, a master isolator switch, kick start pushbutton to start the unit up, voltage meter with a test button.  The internal control circuit for the unit is quite simple with a Arduino Nano as the processor, a dual relay module for switching the batteries and a step up buck converter. I also have a step down buck converter to supply the 5v for the Arduino and relay power.

UM19vB - detail of the controls

UM19vB - Overall look at case

Why are you using old technology Sealed Lead Acid batteries ?

When I first started thinking about these new units I did got through a whole process of using LiPo batteries but their cost is still quite prohibitive. LiPo batteries are quicker to charge and weigh about a quarter the weight of equivalent sealed lead acid batteries but I already rely on a car to transport the motion control rig so the weight of the batteries is not an issue. The price of doing these battery units with sealed lead acid is clearly much cheaper plus I already had a battery charger.  To change to LiPo batteries I would need to purchase a special charger for the LiPo batteries which also adds to the cost.

LiPo batteries with an equivalent voltage and power capacity are not common though I did find 22.2v 12Ah LiPo batteries which would be around $270 for two. This would give me 430W of power which is similar to two 18Ah SLA batteries but at a total cost including the charger components of $500. Whereas I have chosen to make up one 430W unit and one 480W unit for half that cost of changing to LiPo based systems. It seemed more sensible to me at this time. The only real downside that I currently have is that the sla battery charger can only give out 1.5A of power during charging which means that it potentially takes 18 hours to charge one battery. This could be something to look into for the future but since I have 3 of these 400W plus units, I don't think I foresee any issues with not having enough power for a timelapse shoot.

Some downsides of LiPo batteries is they more monitoring while being charged and also need special treatment during the whole discharge process. It is suggested that the LiPo batteries be stored in fire proof bags while they are being charged in case something goes wrong. I have seen what happens when a LiPo battery catches on fire. But this is not the real reason for not using the LiPo batteries. I already have a process for using and charging SLA batteries and the LiPo was to give me no great benefit in the near future.