3.7v vs 7.2v and power output

I’ve always heard that any decent handheld radio or one that needs to output more than 1 watt anyway really needs to have a 7.2v battery pack(lithium models). Many of the cheap chinese radios do not and also do not put out the amount of power they are supposed to be rated for. Also any FRS/GMRS radio that puts out more than 2 watts also uses 4 AAs instead of 3 which would push voltage from 4.5v to 6v. I recently purchased some Motorola RDM2020 radios which use the 7.2v pack @ 1100mah. Motorola recently replaced this model with the RMM2050 and I was surprised that the specs say the battery capacity is twice what it used to be @ 2100mah. I found out that they did this by cutting the voltage in half. It is only a 3.7v pack. I don’t understand why they would do this. I am wondering if anyone has experience with this new model RMM2050 and if the power output has been affected in any way. Perhaps 2 watts is the limit for 3.7v and anything higher needs the 7.2v…anyone have any thoughts on this?

3.7V and 7.2V really has no bearing on the RF output of a radio. Voltage regulators regulate the voltage…and step up or down to whatever is needed (with some losses). 7.2V was really common back in the NiMH days as that was what you typically had to stack NiMH cells to get a meaningful capacity where Li-on and Li-po cells can typically provide the double the capacity in the same or smaller form factor.

One thing to look at is the overall efficiency. First start off by converting the battery capacities into a number that doesn’t care about voltage. Multiply voltage by capacity (in amps) and you’ll get the energy capacity in Watt hours. Example, the RDM2020 battery has a capacity of 7.9 W-hr and the RMM2050 battery has a capacity of 7.8 W-hr. Pretty close. You can take this a step further and look at the spec sheets where Motorola states the RDM2020 can have up to 12 hours of capacity under a 5% Receive, 5% Transmit, 90% standby duty cycle with the 1100 mAh battery and up to 15 hours for the RMM2050 with the 2100 mAh battery.

What it looks like, by stepping down to 3.7V the radio has been allowed to use less voltage conversion (which have losses) making the radio more efficient overall.

1 Like

Yep - concur with this. Not really sure if you need too convert to Watt/hrs. We don’t tend to do that in the UK, just use the Amp/hour figure but it’s a way to compare like and unlike I guess.

1 Like

Ok thanks for the info. Perhaps technology has improved to the point that the higher voltage is no longer necesary to maintain sufficient power output. I’ll take a look at the other Motorola models and see if they went the same route as the RMM2050.

You actually have to convert it because we are looking at the energy capacity of the battery, not the current capacity. If the battery voltages were the same you could look at just the current capacity. Watt-hours are actually a term I hate though (more people understand them though as their electric bills are typically billed per kW-hr) as when you break it into base SI units, you simply multiply by 3600 and you are left with Joules (one Watt being one Joule per second) but that adds a complexity not needed for this comparison.

The other post are basically correct. The ‘amp-hour’ rating can tell you a lot of information with a few ‘depends’ (just like the diapers). Efficiency and “usability” are not ‘equal’ by a long-shot. And ‘advertising’ isn’t to always to be trusted! (More of those diaper things)

I looked at the other Motorola models and all the RM line uses 3.7v but they are also all 2 watt models. Any models with 4 watts or more are using 7.2v but those(RD line) are older design i believe.