I do like this add on extra battery, I have 3 eb batteries, two eb300 and one eb600.
This is a clever design, it just clips into the river 3 plus for extra storage. I have 3 river plus power stations dotted around the house with there extra batteries connected. I find them excellent for running small appliances whilst being charged on solar or cheaper nightly electricity. They all last around 3 days and they are small enough to take with you on holiday.
If you have a River 3 plus or wireless, these extra batteries are certainly recommended if you need some more storage without having the need to replace your power station to a bigger model.

I’ve run a number of tests on my River3 Plus, both with and without the extra battery 300. It’s a really well thought-out and manufactured product that nicely meets its specification.
• When used as a UPS, it switches to and from the incoming mains in less than 10mSec. It’s a relay change-over and on my unit switches in 7mSec. The very small disturbance you get depends on where in the cycle it switches, but should be quick enough to support most modern IT equipment.
• Only tested to 500W load on LED floodlights but battery life is as expected, making a modest allowance for battery to mains conversion efficiency. Quiet cooling fans.
• Very useful mobile app allowing a wide range of settings. Easy to optimise for best battery life. Works on WiFi when available but out in the field your mobile can still link to the unit over Bluetooth – neat!
• Good display on the 3 Plus giving charge and discharge rates and battery status.
• The USB C ports on the 3 Plus and the additional battery unit are high power (100W) and easily power Mac laptops, but use a quality cable that can support 5 Amps!
• From a safety point of view was pleased to find a very low resistance during a PAT test between the mains input earth and all 3 output socket earths. Good design.
• Useful light on the 3 Plus.

Overall impressed and happy with my purchase. I haven’t tested it yet with car or solar input.
Safety note for those using it as a UPS. When the unit is using battery power, the 230V outputs are ‘floating’ and not referenced to the mains input. That means the electric shock risk is low if you only come into contact with one of the output terminals due to a fault in the connected equipment. However, when mains power is present on the input, the incoming Live and Neutral are routed directly to the three output sockets. That means your shock risk is exactly the same as with a standard mains socket, so it is highly recommended that you use a mains supply protected with a 30mA leakage trip – an up-to-date Type A RCD or RCBO. Most UK houses (but not all) have this protection on their ring main circuits.