Solar charging a NiMH cell - Page 1
Forum section: electronics / beginners

author: homebrew
author_rank: Frequent Contributor
timestamp: April 03, 2018, 07:11:38 pm
content: Hi everyone,I just started designing a small sensor node that should run from a solar cell.Thus, I started looking into integrated energy harvesting and charing solutions. A nice looking part could be the SPV1050 from ST (http://www.st.com/en/power-management/spv1050.html).However, they claim to be able to charge *any* type of battery. But how is this even possible? Yes, according to the data sheet one sets an over volt and under volt limit to protect the cell but don't different cell types normally require radically different charging approaches?So what's the deal here? Is ist just the assumption that low current charging won't do any harm to the cells even in the long run?To be specific, this is the cell that I want to charge (or maintain the charge):http://uk.farnell.com/varta/55620303059/battery-nimh-mempac/dp/3519570Any ideas, opinions or experiences on that?

author: TimNJ
author_rank: Super Contributor
timestamp: April 03, 2018, 09:18:55 pm
content: Well, someone can correct me if I'm wrong, but here's my take.Lithium-ion batteries, in particular, have stringent charging requirements because they can blow up if you do it wrong. Other battery types, lead acid for example, are more tolerant to abuse.To safely charge a Li-ion battery, you should precondition the cell, then charge at a constant current until the cell reaches some upper voltage. Once that upper voltage is reached, taper down the current until cell is fully charged. Once it is charged, the controller may continue to provide a small "maintenance" current.In general, I think the lithium ion charge profile can probably be applied to most other battery chemistries. Years back, when NiCd was king, the controllers were relatively primitive, and it was no problem because the batteries could be safely charged without much fuss at all. NiMH is harder to charge by comparison but can still be charged relatively simply. As long as you stay within the charge current ratings of your particular battery, I think the more advanced lithium controllers would work fine.

author: David Hess
author_rank: Super Contributor
timestamp: April 03, 2018, 11:53:46 pm
content: Quote from: homebrew on April 03, 2018, 07:11:38 pmEnergy harvesting applications are such low power that current is automatically limited.

author: homebrew
author_rank: Frequent Contributor
timestamp: April 04, 2018, 03:35:16 am
content: Quote from: David Hess on April 03, 2018, 11:53:46 pmWell, it is advertised to be able to supply a max charging current of 70mA. Given the 210mAh cell, this is actually quite a lot. I didn't see any means to 'program' the max charge current either.

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2018, 03:52:19 am
content: Quote from: homebrew on April 04, 2018, 03:35:16 amThere is an assumption in the design of the SPV1050 that the output current will be limited by the average source power. It is not really intended for constant output current charging.

author: homebrew
author_rank: Frequent Contributor
timestamp: April 04, 2018, 04:03:47 am
content: Quote from: David Hess on April 04, 2018, 03:52:19 amOk, but then I would need another approach to the problem as I cannot guarantee that no-one will mount the device in direct sunlight and have it shined on all day long.Do you have any idea?

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2018, 12:34:17 pm
content: Quote from: homebrew on April 04, 2018, 04:03:47 amI did a quick search and other energy harvesting ASICs do not include a current limit either.1. The easiest thing to do would be to use a rechargeable lithium cell which only needs voltage regulation. A NiMH battery does not strike me as a good choice for this application because of charge termination difficulties and they are not very tolerant of continuous overcharge.2. A current limit circuit could be added. I am not sure what would be best but a shunt regulator which diverts the output current would work with any energy harvester circuit.3. The battery could be replaced with a capacitor and a separate battery charger ASIC or circuit used. This might be just a series current limiter.