July 5, 2018: German start-up BOS —Balance of Storage Systems — says it intends to double the number of its hybrid lead and lithium battery units this year to reach a total of 20,000, chief technology manager Thomas Kündiger told BESB on July 3.

The product, which includes a PV panel, use a 12V lead battery as back-up and a smaller lithium battery to run most of the daily charging cycles so the lead battery does not have to be discharged so much — which prolongs its life.

“We all knew off-grid systems quite well and saw so many lead acid battery systems failing by sulfation, since the owners didn’t charge the systems frequently,” Kündiger said.

“First we developed an all-in-one charge controller, that managed both batteries, and PV. But for many customers’ requirements, this was not flexible enough. So we developed a modular and flexible lithium extension battery with in-built electronics and all protection features, that can be connected to any 12V lead battery.

“When the lead battery gets charged, the charger will switch from constant current charge to constant voltage charge. Constant voltage charge means that there is more energy available than the lead acid battery can use.

“Now our extension battery, connected in parallel to the lead acid battery, recognizes that there is constant voltage charge at the appropriate voltage level and charges the lithium battery with all the extra that the lead acid battery can’t take.

“Once the sun goes down we first discharge the lithium battery, keeping the lead battery and loads around 13V. There, the lead acid battery does not get discharged yet and this way we keep it at a high SOC, while the lithium battery is doing most of the cycles.”

Kündiger used his own home as an example to illustrate how the two batteries work together, with his 2 kWh lithium battery installed with a 10 kWh lead battery.

“Since the lithium battery is used each night and lead acid is back-up for bad weather days and higher energy demands, there is something like 300 lithium and 50 lead acid charging cycles every year.

“This is enough cycle life for eight to 10 years, while investment costs are significantly lower than a lithium battery.”

BOS’s systems are used in mini-grid solar applications including one project in Nigeria, which runs 100 of the systems, a DC minigrid in India, and many are placed with Hymer, the biggest caravan manufacturer in Germany.