When we got a Sunamp Uniq 12 heat battery earlier this year (2020) to store the energy produced by the solar PV panels on the roof, we were a bit surprised to discover that it has no lights, switches or user interface of any kind. You can't tell whether it's charging or not, how much charge it has in it at any time (i.e. can you have a bath or not) and you can't tell it to charge or stop charging. It has no 'Override' control to force it to charge and you can't connect it to wifi or access it with an app. It just sits there doing what it's configured to do. (I believe that Sunamps shipped from late 2020 do now have some rudimentary indicator lights, so that's an improvement.)

(I should probably say that this is, imho, its only failure. It does the job it's supposed to do very well and we're very pleased with it.)

I therefore took a look inside the control box and discovered some pins on the main pcb labelled ground, 5v, tx, rx. That sort of thing... So to cut a long story short I made a little box that does the things I want, and here is a description of it.

Here's the logger in place and working. It's a small black box which attaches to the Sunamp's control box with internal magnets. It has 2 sets of wires coming out of it: a pair which connect to the Sunamp's control box, and a pair which can be connected to a current operated switch so you can monitor how much electricity it's using. Power is supplied by a usb charging cable. It could take its tiny amount of power from the Sunamp's board, but I didn't want anyone saying that it could overload the board's power supply.

The LEDs

• The bottom LED (Red) shows whether its charging or not - on or off.
• The 1st LED (yellow - just above the red one) blips briefly every few seconds when it receives data from the Sunamp. Tells you it's working.
• The remaining 8 LEDs are green and indicate the state of charge, from empty to full in 8 steps. (It's difficult to accurately report a Sunamp's state of charge; accuracy here is a lot better than just 'empty, 50%, full', but I'm still working on a better algorithm.)

Here's the connection inside the Sunamp's control box. As you can see, the logger just connects to 2 pins on the Sunamp's pcb: Ground and Tx. (It actually works just fine with just the Tx connection as long as everything's grounded correctly.) Not shown here, but the cable can go through one of the unused holes in the bottom of the box - I didn't have to make any extra holes.

The logger doesn't just report the state of charge and whether or not it's currently charging and receiving data from the Sunamp, it also contains a webserver which can be accessed from the local wifi network (or from afar with a bit of router setup).

The logger provides 3 forms of information about the Sunamp:

• The current charge and temperatures are displayed at the top along with the state of the heating element, if present.


• The charge, temperatures and current consumption over the past hour, day, week, month or year are displayed in a graph. The red line along the bottom of the graph indicates whether the Sunamp's immersion heater is on.


• As the logger runs, the data is gathered in log files, which can be downloaded as .csv files which can easily be loaded into a spreadsheet for further analysis.

Having made this gadget, we find it very useful; mainly as we can tell the charge state with a quick glance. Of course, if the house central heating system (CHS) was fully automated and working properly that shouldn't be necessary. But at the moment it's not, so it is.

At the moment, we have 2 actual heat sources: a small solar PV array on the roof and a pellet boiler. (And the Sunamp's own immersion heater of course, but we don't use that unless something's gone wrong.) The Sunamp's low power coil is part of the CHS circuit, as is the pellet boiler, so when the boiler runs it heats up the Sunamp and the radiators at the same time. It's a nice simple system. In summer, the pellet boiler's turned off and the solar PV keeps the battery topped up nicely and we don't need the CHS. In winter, the PV produces very lttle power - often not enough to heat the domestic hot water, never enough to even touch the space heating requirements.

But fully charged, the battery can heat the house for some hours (only 2-4 ish at the mo., but that should improve as we add wall insulation, better glazing etc). So in a ideal world, we'd fire up the pellet boiler using a demand signal from the Sunamp when it's, say, 20% charged. The pellet boiler automatically turns itself off when the return temp is high enough and hot water for the CHS would then come from the Sunamp until it fires up the pellet boiler again a few hours later. That's efficient too because if you leave the pellet boiler on and allow it to modulate or cycle , it's horrendously inefficient as it seems to need a bucket full of pellets each time it restarts.

But at the moment, since the Sunamp can only switch demand at either 50% or 10% charged, we just keep an eye on the logger's charge level LED (it's in the kitchen, so that's easy), and turn on the boiler when it's down to 1 or 2 bars.

So....

I'm seriously considering adding a relay to the logger so that it could provide a demand signal which would be activated at a configurable charge level. That would make the whole system automatic. The only thing is I wouldn't be able to call it a logger any more. I'd have to rename it a Sunamp integrator or something instead.

PJR January 2021