After a year long delay I have finally got around to writing another blog post. This post is about the DÆ Battery Pack v1.0 that I completed in 2023. Actually I completed it by mid-year and have since also completed:

An updated battery charger/power supply;
New output board with an interesting discrete class-A design;
An updated phono preamp board with both moving-magnet and moving-coil capability as well as adjustable gain and cartridge loading;
An updated panel meter.

I have assembled all of these into a v4.0 version of the DÆ Phono preamp. There are many changes and improvements to describe in future blog posts.

Now back to the battery pack.

To recap, previous DÆ preamps were powered by a battery pack consisting of six 6-volt sealed lead acid batteries. These packs worked well but I had two main concerns beyond the large size of the pack. The lead acid battery packs are heavy which is not a concern in use - actually high-end audio components are often sold by the pound with heavier components sounding better - right? But for sure heavier is not better when it comes to shipping. A major challenge that comes with weight is constraining the lead acid battery pack so that nothing comes loose when an uncaring courier tosses the equipment around - even with a fragile symbol on the box.

Also lead acid batteries have a limited cycle life especially if they are deeply discharged over an extended listening session. I replaced a couple lead acid batteries already and estimate that the individual batteries may last about two to three years in real-world use.

I decided to design a battery pack using 18650 lithium-ion batteries to solve these problems. The lithium-ion pack is much lighter, smaller and should have a much longer life. Maybe five to ten times longer than the lead acid pack or ten to 20 years.

Much of this information and more is in my January 20, 2023 blog post.

So how did it work out? - in the end very well I think but not without a lot of pain. What I thought would take a month or two at the outside took six months to complete. My main problem was the comparator that compares the voltage on two adjacent 18650 cells. The comparator is part of my charge balancing circuit. My first attempts used an Onsemi NCS3402DR2G comparator. This is a dual “Nano-power” comparator which only requires 470nA supply current/channel to operate. That is less than one half a micro-amp! Sounds good right! Except when you want to disable the balancing function when the battery pack is being discharged and you don’t want waste any power in the balance resistors. With such a low supply current, if you just look at the comparator it is “on”.

Other prototypes used an ST Microelectronics TS3702CDT comparator. This requires a supply current of a whopping nine micro-amps per comparator but also has diodes protecting the inputs. The diodes are tied from each input to the positive and negative supply which sounds reasonable. That is until you want to disable the balancing function by removing power to the comparator supply pins only to have current flow into or out of the input pins through the protection diodes to “power” the comparator. Result - more magic smoke released from the device or traces around the device.

In the end I resorted to a much more “jelly bean” comparator, the Onsemi LM393DR2G which requires a staggering supply current in the milli-amp range and has BJT inputs without the same protection diodes but at least I could reliably disable the balancing function when required.

The final circuit (the schematic in this link is a five cell portion of the battery pack and the balancing circuit is on the third page) has many other improvements including reverse polarity protection in case an 18650 cell gets installed backwards by accident.

Another custom feature of the DÆ Battery Pack is the LEDs to indicate a cell is Ok status or the balance active. These LEDs are under control of the companion battery charger and are only active while the pack is being charged. Two LEDs are located next to each cell. The first LED is illuminated when the voltage of an individual cell is in a valid range between 2.8 and 4.275 volts. If the voltage on any cell is below 2.8 volts, further discharging of the pack will be prevented. If the voltage on an individual cell is above 4.275 volts, further charging of the pack will be inhibited. A faulty cell is identified when it’s status LED is off during battery pack charging. This would require the pack to be disabled and the cell replaced. The type of repair can be easily accommodated by a pack that uses cell mounting clips instead of the much more common spot welded cell connections.

The second LED indicates when cell balancing is active. Each cell has it’s own balance resistor that is connected across the cell when it’s voltage is more than a small percentage greater than either of the two neighboring cells. This type of cell balancing is active during the whole charging cycle and not just at the end when the cells are nearly fully charged.

The specifications for the pack are:

Eleven 18650 cells;
36.5 to 45.1 volts;
Current limited to 2A by fuse;
3500 mAh cells. Should power preamp for at least six hours. These are currently some of the highest capacity cells available. Other cells with either less or greater capacity (if such a thing becomes available) can be installed;
Battery Management System (BMS) with over/under voltage protection and charge balancing;
Individual cell status and balance active LED.