A step by step approach to designing and building a charger monitoring solution for off-the-shelf mobility chargers. This blog was inspired by this thread on the WheelchairDriver.com forum.
We start with an overview of the issue and a review of the charging processes.
For those of us who genuinely care about getting the best out of our electric wheelchairs virtually nothing generates more confusion, fear, uncertainty and doubt as the humble lead-acid battery and the charging thereof.
It is a fact, however much debated, that lead-acid (LA) batteries are pretty poor for mobility purposes if you are a serious user (i.e. more than a couple of miles a day). A serious user will go through a good set of batteries (MK, Odyssey) in as little as a year and a rubbish cheap Chinese set in 6 - 9 months. And worse if a tiny 40Ah set is fitted instead of the full 73Ah set most chairs can take. Even if you are a light user you will experience a loss of performance after only a few months, because the standard mobility charger murders batteries. Most stop charging (green light) far too soon and generally charge at the wrong voltages, usually set at some compromise for the different LA electrode types (SLA, AGM or pure Lead) which all require different charging regimes. Truly serious users have migrated to Lithium Ferro-phosphate, LiFePO4, batteries, which are far superior and more cost-effective than LA when looked at over a 5 - 10y period. But that's a story for another blog.
The basic charging regime for LA batteries, and what should happen with a good charger, is this:
Constant Current (CC) Phase:
The charger outputs a voltage just above the desired maximum battery volts at its highest but controlled output current (typically 8A or 12A for a mobility charger through the joystick charge port, but could be 40A or more for a directly connected fast charger). As the battery charges its voltage rises until it is close to the desired maximum voltage.The duration of this phase is determined by how discharged the battery was and how large the charging current. At some voltage the charger switches to the...
Constant Voltage (CV) Phase:
The charger output voltage is now fixed at the desired maximum voltage. The nearer the battery voltage gets to it the lower the current drawn. Eventually the current drops below a preset amount, the charge termination current. At this point the battery is deemed to be fully charged. Typically the termination current is a few tens of milliamps, maybe 100 - 150mA, for a 73Ah battery. This can take 8 to 12 hours or more.
Float or Maintenance Phase:
The charger output voltage drops to a lower float voltage (again specific to battery type) which trickle charges, at a very low current, the fully charged battery as it loses charge (self-discharge). This phase is of little use for most mobility users as it only applies to batteries on charge for days or weeks, and most wheelchairs are charged daily overnight.
Typically mobility chargers fall down in a number of areas:
- The current in the CC phase isn't constant (often because the charger's maximum output voltage isn't high or stable enough);
- The maximum charge voltage in the CV phase is incorrect for the battery type and/or is unstable;
- The green light (charge finished) comes on at too high a termination current;
- The float phase, if present, starts too soon and/or at the wrong voltage.
The graph below (an amalgam of two graphs generated from a PL8 charger by John Williamson, owner of wheelchairdriver.com, which can be found in the thread linked above) shows a good charging process. At the point where the graph finishes, however, the current hasn't dropped to the termination value of 130mA - that took another 3 or so hours. Current, the green trace, is on the RH scale, whilst voltage, black trace, is on the LH scale.
So, in principle, to fully monitor the charging process of a mobility charger, we must not only track the charging voltage, but track the charging current as well.
The next entry on this category will look at the design requirements for the charge monitor.
Feel free to comment below but be nice, keep them relevant, and on topic, otherwise I'll remove them!