The picture below is a rendering of one half of the DÆ Battery Pack v1.0. This improved battery pack will replace the lead acid battery pack used to power the analog electronics in the DÆ pre-amps.
DÆ Battery Pack v1.0 lower bank with five 18650 cells. Battery side.
The battery pack I am currently using consists of six 6-volt sealed lead acid batteries as shown in the picture below. The lead acid batteries are the dark grey boxes that take up most of the interior of the enclosure.
Interior of DÆ Phono Pre-amp v3.0. Note the phono preamp electronics on the rear panel, battery pack with six sealed lead acid batteries and “Sort of Analog Panel Meter” on the front panel.
With the aim reducing the size of the finished product, I am designing a lithium-ion based battery pack around 18650 cells. The new battery pack is about one third the size of the lead acid based battery pack for the same or higher capacity. Higher capacity translates into more battery-powered listening time - always a bonus. The lithium-ion battery pack is also lower weight which is a mixed blessing. It will be less expensive to ship pre-amps with lower weight and a smaller size but every audiophile knows that the heavier the equipment the better the sound :).
The concept that heavier audio electronics sounds better may actually have some basis in fact because a heavier chassis may help reduce microphonics in sensitive circuits like the input stage of the phono pre-amp - but I digress.
The most common way to build a lithium-ion battery pack is to spot weld 18650 cells together with thin metal strips. I built a battery pack this way and it works well but I didn’t like the fact that replacing a damaged cell in the middle of the pack would be extremely difficult and the whole pack would likely be scrapped. I wanted a way to detect and replace individual cells if they go bad so I designed a custom battery pack using clips to retain the 18650 cells.
Lithium-ion batteries are much more particular about how they are charged and discharged when compared to lead acid batteries. It is difficult to damage a lead acid battery by over or undercharging. By contrast, there is only 0.2 volts difference between the voltage of a fully charged 18650 cell, about 4.1 volts, and a voltage of 4.3 volts that will damage the cell due to overcharging. Also the 18650 cells are damaged if discharged to too low a voltage. A battery management system (BMS) is required to protect the cells. There are commercially available battery management systems but I wanted to design a custom BMS tailored to my needs. The image below shows the BMS electronics on the bottom side of one bank of the DÆ Battery Pack v1.0.
DÆ Battery Pack v1.0 lower bank with five 18650 cells. Battery Management System Side.
Please try the 3D model viewer below to examine the lower bank of the DÆ Battery Pack v1.0 from any angle. Press the play button to load the 3D model.
Commercially available battery management systems perform a few functions. Each cell has over/under voltage protection. I am using the Diodes Incorporated AP9101CAK6-AUTRG1 integrated circuit which is designed for this purpose.
A BMS may also include a way to balance the voltage and therefore the state of charge of the individual cells. Balancing the cell voltage across a battery pack can be critical to the capacity and life expectancy of the pack. As a thought experiment, consider assembling a battery pack from 18650 cells with unknown and random initial states of charge which translates into random initial voltages. When a charger is connected to the pack, the same charging current will flow through each cell assuming a series connection. The cell with the highest initial voltage will reach it’s maximum fully charged voltage first and initiate a pack “charge inhibit” to protect itself. This will stop charging the other cells which are only partially charged; this reduces the overall capacity of the pack.
Battery balancing is used to better balance the charge on all of the cells. From my study of commercially available battery management systems, balancing is typically done by connecting a load resistor across a cell when the cell voltage goes above a certain high value, say 4.2 volts. The load resistor will slow the charging of a particular cell and give the other cells a chance to catch-up. This helps to balance the voltage and charge of the cells but only as the cells approach a full state of charge. I though I could do better and besides I couldn’t resist the opportunity for some creative circuit design.
The circuit (this link is the whole BMS circuit and the balancing circuit is on the third page) I designed balances the cell voltage through-out the charging cycle instead of only right at the end. Here is a description of how it works. Consider a back pack with only two cells. If the cells are balanced the voltage on each of the two cells will be equal. This is true if the cells are fully charged, at nominal charge or nearly discharged. At any state of charge, a load resistor is connected across a cell is it’s voltage is greater than one half the voltage of the two-cell battery pack by a certain amount. This way the balancing occurs through-out the charging cycle (or discharging cycle also if you like) and not just at the end.
The circuit balances two cells at a time but if the same circuit is repeated across a battery pack with more than two cells the whole pack should be balanced. In other words if Balance Circuit #1 balances Cells #1 and #2 - and - Balance Circuit #2 balances Cells #2 and #3 than all three cells will be balanced. Mathematically if a = b and b = c than a = b = c etc. To balance the whole eleven cell DÆ Battery Pack v1.0, ten balance circuits are required.
At least that is the theory.
Bench testing of this design on a breadboard was shown to effectively balance two cells but testing more cells on a breadboard is cumbersome. As a next step, I designed the printed circuit boards for the DÆ Battery Pack v1.0 but it is mid-January so I am waiting until the end of the Chinese New Year to send the PCB to the fab shop.
Don’t you just hate it when statutory holidays get in the way of the progress on your projects :).